Why are there still no extensions for Chromium on mobile?

For exactly the reasons you state - Google doesn't want ad blockers in their browser.

Wonder why they haven't nuked them on desktop Chrome then, where extensions are a plenty?

Is mobile a much more juicy fruit for their advertisers, or is it like said elsewhere in here more a technical thing?
- Wonder why they haven’t nuked them on desktop Chrome then, where extensions are a plenty?
  
  Extensions are so popular on PC browsers, that Google could really jeopardize their dominant market share, if they were to completely remove extension support. The press would be on it for weeks and there could be a real hit on user numbers.
  
  I think that Google rather tolerates the small number of users who use extensions and doesn't want bad PR for Chrome on PC.
  
  But I wouldn't be surprised, when Google tries this in the future, when their browser market share is over 90%.
  
  The game is different for mobile though – here we have a much bigger majority of unexperienced users who likely have never heard of browser extensions or such possibilities as easy-one-click-installation of ad-blockers.
- Probably because the cat is already out of the bag there. Hard to reign them back in and they'd have tons of bad press if they do that.
fair point!
But what I don't get is why doesn't Microsoft or someone large like that bundle extensions into their browser. I know Samsung has app based ones but I wish it could be built in and have ublock origin etc

I just use Firefox and ublock anyways, since chromium has a neat monopoly on the web. A monopoly on browser tech is terrible for the web.

I was an avid Firefox user back 15 years ago, when the Windows program would gobble up all the ram.

Chrome was so light and quick, like everybody else, I switched.

About 5 years ago, a new Firefox came out and I gave it a try and never looked back. So many neat plug ins! And uBlock on my phone!
Unfortunately, I've also found Firefox to have a few breaking bugs that make the experience basically unusable in mobile.

Things like the browser engine(?) crashing, causing the page to be soft-locked on a blank page, and running poorly compared to its other counterparts.

Which is rather a shame, since it would be much more usable and promising if they fixed those issues.
- Weird, what device? I have a fold 4 and it's been pretty solid, save for this occasional URL bug.
- I find it strange that you're having this issue. I haven't had this issue on my OnePlus 3T, Note9, or Pixel 7 Pro using Fennec F-Droid or Mull (FOSS forks of Firefox)
- No such issues here on my Fold 4. Do you have an example of a specific website where it crashes on?

Outside Chrome, the answer is that the extension support is a very big and fragile (hard to maintain) patch for Chromium.

Here's a list of browsers with extensions:

Kiwi and Yandex browser support most Chrome extensions
Firefox, Mozilla's Reference Browser and Firefox forks support most Firefox extensions, but you need to make a "collection" if you want more variety than the default list
Samsung Internet supports some content blockers as app-based extensions
SmartCookieWeb, Berry Browser, Sleipnir support userscripts
Many other browsers also have some form of tracker and/or ad blocking

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Firefox, barely. There are like 18 extensions unless you run Nightly and jump through a bunch of hoops.

I am puzzled that this situation has persisted for years now. It seems like Mozilla doesn't really want extensions on mobile either.
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I use Kiwi out of necessity. The modern web is unusable without some way to block ads, cookie popups, and sticky elements. It's especially worse on mobile because every other website wants you to use their app.
I'd strongly prefer FF, but since they yoinked the Bypass Paywalls extension, I've been taking a look at Kiwi. Eventually once Manifest V3 goes though I'll want to move to FF regardless, so I'm hesitant to consider Kiwi as a permanent solution though.
Yandex browser also has chrome extensions and is actually supported by a big company (that may or may not be affiliated with Russian government, but whether or not that affects you personally depends on your threat model).

The only Chromium based browser for android that I know can run extensions is Kiwi, most of webstore extensions works well on it.

I use samsung browser (chromium based) it have some extensions and built-in adblocker.
I love Kiwi Browser. Only downside is the developer seems to update it every 5 months or so.
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Extensions were a thing well before Chromium and I'm sure even the developers wanted them, so we got extensions.

There's never been a precedent for extensions on mobile and Google knows they can get away without, because if they do they'll have a hard time taking it back. And they really don't want everyone to have ad blockers on mobile because they know mobile ads is what brings in the cash, and they know mobile is one of the places where ad blockers would be the most useful and effective because the ads are so intrusive and annoying.

firefox

Kiwi supports custom extensions

@zephyr I use kiwi browser. Don't know how secure it is though. Google doesn't want ad blocker on their browser it seems.

I just block ads via dns (next dns, set in android network and internet settings)