Why are many businesses in democratic nations largely undemocratic?

Depending on who you ask, Capitalist countries aren't truly democratic because of this, and Capital's influence on government.

I'm proud of you for asking a difficult question that you won't get a satisfying answer to.
Its almost like asking "why doesnt everyone share cars?" You probably aren't using one all the time, they're expensive to maintain, why not distribute the load to society and just have fleets of cars you borrow whenever you need one? Like a vehicle library.
Some people will love this idea, it would work very well for them. Some people will hate this idea and rail about how its the death of freedom and personal choice. And some will very rightly wonder "why are we talking about cars? Trains solve this problem 1000x better!"
Privately owned business is a problem, and a major component of the problem is that petite bourgeoisie small business owners believe they're part of the broader "business class" which doesn't exist. They're exploited smallholders who serve the interests of the truly rich and powerful by ideologically aligning with them against workers, whom they universally believe are too stupid, selfish, and myopic to properly make decisions for themselves or anyone else.

Why would you expect them to be?
If my family starts a restaurant and hires additional workers to, for example, help clean, bus tables, wait tables, and so on, I think it would be kinda weird to share the decision making between all employees. It makes more sense for employee owned corpos, but most small businesses have an owner or owners whose main job is steering the business.
- It would only seem weird because you are used to it. Not because it is right.
  The person "steering the business" should be in that position at the behest of the workers. If you can't run a business literally by yourself, you should share power with the people hired to help as if you would a partner.
  
  Yeah I just don't agree with you.
  There is certainly a broad set of circumstances where businesses can share ownership between employees, but that does not mean there are not other circumstances where work is done purely in exchange for money, benefits, or both.
  If you and 4 friends want to start a pizza shop, cool do it democratically. If I do a business selling a product all myself and every other Sunday I pay someone to come lick my stamps for an hour so I can spend time with my kids, that person is not an equal partner.
  Edit: to be extra clear, democracy is based on the concept that people are all functionally equals in capability, information, and perspective. Basically that countrymen are homogenous. Inside a specialized enterprise of any kind (especially small ones) this need not be true.
  Edit 2: or if that's insufficient and all businesses must be democratic, then I necessarily must be allowed to hire based on whatever criteria I so choose. Work ethic, want to keep the company aligned with my interests, religion, ability, height, anything. That's the only way to guarantee a homogeneous pool and may also be the democratic will of the group of people who begin the business.
- Because I would expect people in democratic nations to value democracy and see it as worth exercising in business. This is in part as I see democracy as a formal way of referring to being open to discussion of opinions and ideas in organizing any group.
  Why would you want to be part of any group that may reject open discussion of its organization?
  
  In a democracy you vote on what happens with a shared resource that belongs to all of you, like a country. If a business has several owners they might steer it democratically, like a family business deciding together what to do. But if that business hires employees, the employees don't vote, because it's not their shared resource, so why would they have power to decide on it?
  Of course that doesn't preclude open discussion. Many businesses decide together with their employees, it's just based on discussion and exchange of ideas, not on voting. Why would you hire an expert and then vote among employees instead of letting the expert decide on their area of expertise?
  
  The fact is not everyone has an informed perspective and in business there are very good reasons to not give the entire staff access to finances or company secrets.
  As not all employees have the same information then not all employees are going to be able to see that larger picture and thus not giving the guy with 5% of access to the picture the same say as the guy with 95% doesn't make sense.
  
  Because you can easily swap companies, but you can't easily swap countries.

There isn’t an inherent value to making all businesses democratic because very often most workers have no idea how the larger company works as a whole.
I work for an import company. My union warehouse steward is constantly judging the financial health of the company based on the volume of boxes he is shipping. The problem is he has no idea the relative value of those boxes so while he’s bemoaning we sent out 1/4 of the number of boxes on Tuesday that we sent out on Monday he’s missing that the total value of Monday’s sales were 3x Tuesdays. In 5 years of working with the guy he has never wrapped his brain around this. Our company would be much worse off if he had a say in how it works because he simply cannot see the larger picture as those skills were never developed. This is not uncommon and I myself have been the guy who cant see that larger picture in other roles.
Should the janitorial staff have equal says as to the executives in how funds should be allocated? Do we recognize that not everyone has the same skill set and level of skill as others?
- You can use all of these same arguments to argue against democracy in nations, too. The average person has no idea how the nation works, all of the ins and outs of government, to say nothing of the larger global stage. Clearly what we need is a monarchy!
  
  That's a false equivalence though as private businesses and governments are not the same.
- Should the janitorial staff have equal says as to the executives in how funds should be allocated?
  Given their propensity for allocating the funds to themselves, probably.
  
  Yeah that's not as common as people who have never run or managed a company or budget think.
  The reality is your maintenance staff isn't going to have the skill set to make rational judgements outside their expertise.

Because as of yet the means of production aren't public property. So the people who own them get to decide the structure of production and they decided we don't get a say in how they are used.
- Do they need to be public property or do they need to be in the hands of those working there? I’d be more inclined towards the latter as in most cases the public as a whole is not going to have an informed or educated perspective on how specific jobs/roles/companies should behave.
  
  A mixture of both, with the public holding primary power. There's benefit to people having a better perspective on that which surrounds them immediately, but as industry gets more complex and advances ever more, that "immediate" shrinks more and more as a proportion of the overall production process.
  
  Those are so similar to each other in comparison with capitalism that at this stage, we mostly use the same words to describe both.

Capitalism is antithetical to democracy. Capitalism left unchecked will eventually lead to fascism.
- The first sentence is not true. The second sentence is absolutely true. It is funny how that works.
  
  The first one is true.
  In democracy, the people rule society
  In capitalism, the rich rule the economy
  The economy always rules society
  In a capitalist democracy, society serves two masters. Both opposites. It's inherently unstable because it's self contradicting.

Because you are not paying enough attention:
a Joint-stock-company is by definition democratic. The shareholders are meeting reguarly and voting who get's to sit on the board, can fire the CEO and so. That doesn't apply to the workers, yes, but between the owners it kind of is democratic.
Yes, I know that many tech companies have this strange divide between "voting stock" and "non-voting stock" and founders, who still are in control without owning the majority of the stock, but that is an american thing and not legal in many parts of the world
there are also many ways to ensure democratic collaboration within a company. Look up the german "Betriebsräte" f.e.
there are also many cooperatives around there who are owned by their workers
and there are many state-owned companies around in democratic nations
- Because you are not paying enough attention:
  I appreciate the examples provided but disagree with your opening, and would suggest the same of you. I specifically said "many businesses" and "largely undemocratic" as I was aware of most of the examples you gave beforehand.
  In particular I don't view the joint-stock model as sufficiently democratic due to what you already acknowledge, i.e. limited to owners/shareholders.
  Regardless, appreciate you bringing to light "Betriebsräte", as I'll have to look into that.
  
  Democracy is "owned" by stakeholders, and those stakeholders are the people. So it makes sense for them to have a say in how government works.
  A company is owned by shareholders, and they take all of the risk for the company. An employee shows up and gets paid, with none of the downside risk (their paycheck won't go negative), so the employee isn't a stakeholder. Therefore, shareholders make the decisions, not employees.
  In some structures, employees are the share holders and thus help make the decisions.
- "kind of democratic between the owners" is just oligarchy. still not democratic.
  
  That's like saying the foreigners not having a vote is being not democratic though. Because 100% of the owners have voting rights not only a few.
  I think what you intend to criticize is the fact that owners and "employees" can be separated, right? If yes then I'm with you.

Well, there's nothing inherent about democracy. Nothing about reality inherently forces society towards a democracy.
Our democracies are just as socially constructed as our workplace structures. One of them (society) we've managed to make democratic. The other (businesses) are much smaller, and larger in number, and thus harder to influence overall as a system, thus it's taking us much longer to push them towards democratic structures as well.
- You get where I was going with this! It's exactly that constructed form, and the supposed favoring of it, that led to my asking this.
  If a society claims to embrace democracy, but doesn't extend this to the organization of its businesses, how much do they embrace democratic values?
  
  Why would a privately owned business need to be democratic? What is the advantage of giving all employees an equal say regardless if theor skills and understanding of the business from a business perspective rather than a moral one? I cannot see any reason to give the guy who is not permitted access to all the secrets and finances equal say with the folks that have this access.
  Have you ever owned, managed, or run a business before?
  
  how much do they embrace democratic values?
  Not as much as we'd like, unfortunately. A lot of people are DINOs. (Democracy In Name Only)
- Violence tends to speed things along.
  Just sayin.
  
  Not in this case it does not.