[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.