Why Steam can be considered a monopolistic platform?

It's not a monopoly. There are numerous other stores like epic, GoG, and others. Consumers have other options.

Because a monopoly is a company that operates in their market unopposed. In this instance, it's not Steam's fault the opposition kinda sucks (or doesn't quite aim to be a direct competitor in the case of GoG) but the argument is still there that Steam is sitting as the only distributor for PC.
Personally, this is why I keep wanting to root for GoG, Epic and such. Monopolies are dangerous to consumers and the markets they operate in. Right now, Steam is being surprisingly effective at remaining a "good guy" but there's a lot of concern even among Steam fans of what the landscape will look like in a post-Gaben world. Setting the PC gaming market up to have Steam as the only option when that inevitably comes to pass (touch wood that that's no time soon, of course) could spell a certain level of disaster in a world where the anti-monopoly law-makers have shown to not really care about upholding that standard.
Edit: missed words. Never type when struggling to keep your eyes open kids!
- There is little to no concern about steam, you can't even say that they aren't great (their launcher is horrible for example).
  
  Steam has the worst launcher, except all the others.
  
  There's absolutely concern about Steam if you're looking at the discourse. Personally, I hate the Steam launcher and have kept having problems with it ever since they changed the design to be more Baby's First like everyone else has done (not just taking about launchers here) but the Steam launcher is still better than the others, which is infuriating.

I live in constant dread of the day Gaben kicks the bucket and Steam is carved up by bloodsucking profit-chasers

Thank god nobody from this comment section was involved in antitrust cases against Microsoft.
- M$ did hella shady, monopolistic stuff (patent theft, market manipulation, very likely corporate espionage, and certainly most visibly prefferential treatment of their own software ecosystem and sabotage of third party software on their platforms) to create and enforce market dominance. Unless Valve has been doing something I'm unaware of to kill other platforms, they're not really similar situations.
  
  Valve runs a couple of online casinos that target children specifically, not sure we should be arguing who’s worse here. I think Steam is a clunky piece of software that’s popular mostly because everyone else missed the moment to start competing and Valve gained monopoly unopposed. Other viable competitors tried and failed at even gaining a foothold and are relegated to small niches because it’s impossible to move people who amassed content libraries over the years. Valve skims 10-30% of an insanely large volume of transactions and should be held to a much higher standard. You’re ignoring all of the warning signs because they didn’t screw you over yet.
- The monopoly case against MS was bullshit. They had all kinds of bad business practices to go after and they decided to go after them for including a web browser in the OS. They fucking made the whole process a waste of time.
  
  Pressure on their web browser monopoly was necessary because IE6 was stifling entire industry. From a legal point of view it’s not illegal to be a monopoly but to abuse that position so there isn’t that much you can do about it, especially in the US. Going after operating system or office suite monopoly should have been done but matters less and less these days.

Did you mean to have more in this post? I'm not sure I fully understand. I'll remove if there wasn't more you were trying to say
- I assume he’s asking “why can’t steam be considered a monopolistic platform”

For them to be prosecuted as a monopoly, or be considered one legally, it would have to be shown that they achieved or maintain their dominant market position by preventing or undermining competition. Say by having a bunch of exclusivity deals to keep big name titles only on their storefront, or by buying out any competitor that gained traction.
Monopoly isn’t about being the biggest seller in a market, it’s about being the biggest player in the market by undermining competition and restricting commerce.
Edit: want to clarify there is a distinction between the legal meaning of monopoly (see the Sherman anti trust act and other subsequent laws and rulings) and the colloquial usage (Only seller in the market). Steam is nether.
- Steam is nether.

A lot of people seem to think that a monopoly has a much, much more direct and literal definition than it actually does.
The definition the FTC uses is:
Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors.
That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
Courts look at the firm's market share, but typically do not find monopoly power if the firm (or a group of firms acting in concert) has less than 50 percent of the sales of a particular product or service within a certain geographic area.
Some courts have required much higher percentages.
https://www.ftc.gov/advice-guidance/competition-guidance/guide-antitrust-laws/single-firm-conduct/monopolization-defined
I have a bachelor's in Econ.
The people that run and advise the FTC have PhDs.
(Well, at least untill Elon and Trump put the fucking Shark Tank guy in charge, or something like that.)
Generally speaking, a monopolist is a single entity that has captured a huge chunk of an otherwise varying and well differentiated market, if your market is closer to the theoretical (ie, not real) idea of perfect competition, or if you're talking about a consolidated market with only a few major players, the monopolist has at least 50% of the market, though, depending on other factors, that line may be drawn at up to 75% ish.
Different specific situations, regions, laws, etc. establish differing specific criteria... but the idea is not that a monopolist is defined by being literally 100% of the market.
That situation would specifically be referred to as a 'perfect monopoly', and like 'perfect competition', is basically theoretical only, or a situation where you're looking at something like a local public utility or some kind of government/state entity.
In actual mainstream academic and legal usage though, a monopoly is a single entity in the market has a very outsized market share when compared to any other market participant, such that its actions alone can very significantly affect all other market participants.
...
Now... when it comes to Steam... a whole lot of the arguement is 'what is and is not the market, what constitutes its boundaries?'
If you define it as just 'PC video games', then sure Steam likely is an effective monopoly.
But if you define it as 'all digital games', then no, not even close, the Google Play Store and Apple Store are responsible for far more digital game downloads than Steam, way waaay more games are mobile games than PC games, if you go by yearly or monthly downloads, or market share.
It gets even more complicated with cross platform games.
Ultimately, it would be up to a lawsuit, lawyers, judges, industry experts, to argue all of the specifics of exactly whether or not its legally valid to formally classify Steam as a monopoly that would need to be broken up or penalized or regulated in some way, and a huge part of that legal battle would be based around differing definitions of what exactly Steam is a monopoly if, and whether those precise definitions are valid.
...
'Other options exist for consumers' or 'they don't have a perfect monopoly' is not a valid arguement against Steam being a monopoly if Steam facilitates 85% of PC game sales, and the other 15% is split up between 10 or so other digital store fronts.
If that is your rubric for 'what is the market', then yeah, Steam is a monopoly.
But, if your rubric is 'all digital games', then no, Steam is just a large player in a market with other larger players, other slightly smaller players, and many other very small players.
...
Beyond that, a huge part of legally being determined to be a monopoly is unethical/illegal behavior of the 'monopolist' being used to obtain or maintain their monopoly.
In Steam's case... I think that would be pretty hard to substantiate, its more so just that Steam had the idea first, expanded upon it quite a lot, and no one really bothered to try to compete with them on the same level untill basically the Epic store, fairly recently.

Mono means one.
There are multiple (more than one) other stores available.
- Mono does mean one, but that's not the legal definition of a monopoly.
  
  I was simplifying it for OP.
  I'm also not a lawyer.
  
  but, steam does not fit the definition of a monopoly under US law, as that would require them to have “restricted commerce or trade” to achieve their dominant market position.
- poly means many
  so if both mono and poly are in monopoly, why do you only pick mono, or why does only mono matter here?
  
  The word for a market dominated by only a few very large players is oligopoly, not... polyopoly.
  Not saying you're saying that, just saying.
  ...
  As to the etymology...
  Its derives from Greek.
  A monopoly has one (mono) influential seller for many (poly) consumers.
  An oligopoly has a few, wealthy (oligo, as in oligarch, oligarchy) sellers for many (poly) consumers.
  Importantly, in Greek, poly is closely related to polis, meaning basically 'all of the people/citizens'.
  This is also where English gets 'Politics' from.
  ...
  Also, I wrote a whole other comment, but the mere existence of any competitors, no matter how small... doesn't mean you aren't a monopoly.
  Its just means you aren't a perfect monopoly, which basically never exists in real life, outside of public utilities.
  If the rubric for 'is it a monopoly?' was 'do any competitors exist?', then basically no company that's ever been broken up or regulated for being a monopoly was actually a monopoly.
  
  Because that's the way i decided to dumb it down. Apparently it wasn't dumbed down enough.

The nice thing about Steam, is that it's "too big to clamp down":
People used to 🏴‍☠️ on the high seas, for many reasons.
Steam came up as a "single point of sale", at the same time as Netflix was doing the same for movies and series.
Over time, companies tried to carve out chunks of the pie, restoring some of the original fragmentation...
...but while Netflix has been torn to shreds of its former glory, Steam is still the main "single point" for games...
...with a "single point" DRM
Steam's DRM only exists because game updates keep coming out with constantly updating DRM versions. The moment Steam tried to act against its clients, and they decided to leave Steam, every Steam game copy at that moment, would get cracked all at once.
Maybe EA, MS, Nintendo, Sony, etc. don't see that as a great thing... and that's why they've been setting up their own stores... but I think it's AWESOME! 😁
- Most single player steam games are cracked anyway. The real danger of steam is the reliance on it for most multiplayer games. Though if it were to get particularly nasty I imagine adding aftermarket multiplayer functionality would probably be in the realm of possibility. If private WoW servers are a thing, it stands to reason that the same can be done with a lot of other games.
  
  Steam supports different multiplayer server modes: Steamworks Multiplayer
  Some games already use P2P, or provide servers for the community to run, so only the private servers would need replicating. Even in that case, I'd argue that having "some" common API, would make it easier than chasing around everyone's different implementations.

The problem with judging Steam as a monopolistic platform is whether it uses its market position to maintain its monopoly or not.
Valve doesn't really engage in vertical integration. There are a few games that Valve makes as a first party exclusive, but nowhere near other competitors like Nintendo or Activision Blizzard. There also isn't a gaming engine that ties to Steam directly; the closest is Proton but that isn't required.
Valve doesn't seem to seem to require onerous requirements on third party game studios to publish on Steam. Outside of banning ad-supported gaming, Valve doesn't seem to demand preferential treatment.
Valve could easily become a problematic monopoly, but it isn't there yet.

It might be. It hasn't been tested in court.
I lean towards 'no' because I do not see moves on their part to actively attack other distributors, but I admit I have not done research on this subject.
Based purely on having used many other distribution platforms, I think they (Valve) just legitimately have the best service currently. Everyone else either kinda sucking (GOG, as much as I love them), or really sucking (EGS, Origin, UPlay, etc), and losing to you in the market, doesn't make you a monopoly.
- I think they care about their customers just about as much as they care about making money, and aside from GOG, the competition simply does not. It's a pretty good demonstration to how capitalism has failed us, to be honest, because any of those competitors would have been able to compete if they hadn't treated their customers like shit.
- I lean towards 'no' because I do not see moves on their part to actively attack other distributors
  That doesn't matter. There's a difference between having a monopoly and abusing it to distort the market. It's the abuse that's illegal, not the monopoly in itself.
  
  There’s a difference between having a monopoly and abusing it
  Sure, but whether Valve fits the definition is debatable. Being highly dominant does not automatically make something a monopoly. At best you could call it an imperfect monopoly/ imperfect competition, because substitutes absolutely do exist, but they're not mostly close enough to be truly competitive. It's also important to factor in that 4/5 of the largest games on PC are not even on Steam at all: Fortnite, Roblox, Minecraft, and League of Legends. PUBG is the only one of the top-5 that's on Steam.
  
  Under US law, yes it does matter, that’s what makes something a monopoly under US law, otherwise it’s just a dominant market position.

Ehh. They haven't really abused their position. They're popular.
It would be something else if they were buying up competitors like Facebook and Google do. Part of how they maintain their dominance is buying out anyone that competes. Notice how Google kind of sucks nowadays? They're not really competing on merit anymore.
But at the same time, steam could turn around tomorrow and be like "mandatory $39.99/mo subscription fee" and it would have an outsized impact on the sector.

Don't take this from me :(