[Dah00n]

Because as it is now Chrome (read: Google) can dictate how the web works and they do. If you use a Chromium based browser everything just works and happy people doesnt often complain or see the problems. So using something Chromium based makes you part of the problem. Apple isn't much better though so there aren't many options left...

[cheschire]

What prevents forks of chromium from sharing amongst each other? I don't think google can gatekeep to the extent you seem to be implying. I'm pretty sure Brave and Opera and MS Edge can share changes they've made amongst each other without going through the original repository first.

[Dah00n]

Nothing stops them sharing but they aren't big enough (and in some cases not interested in) changing the web standards.

Think about it like in this oversimplified/stupid example: A "fix" or change is implemented in Chromium. In the long run (intended or otherwise) it turns out it moves pixels slightly differently than the way Firefox does it. If almost all your visitors use Chrome you have to design your site to be perfect in chrome and you might do so in Firefox. Now you have sites that look as intended in Chrome but maybe look as it should in Firefox. This make Firefox users use Chrome more.

Now Brave et. al. is part of the problem, helping drive the only real competition out of market (and killing their only way out should they some day need to change engine).

In extremely complex code this is very hard to not be a part of and a company like Brave is way too small to fork Chromium for long if at all.

[SXX]

You underestimate how much of effort and money it takes to maintain fork of a browser. Chromium is project as big as Linux kernel (if not larger), but it's primary developed by Google emplyees so they have huge influence on direction of the project.