No motivated group has the experts to maintain a modern rendering engine (tens of millions of lines of regularly evolving code) and Google can reject any upstream changes that group wants to make because Google engineers are the gatekeepers for 97.498% of Chromium code. So, if your agenda has anything to do with web content, you take what Google hands you. Of course, if you just want to diddle in the browser UI, sure fork it, that's the Brave and Opera approach, but hardly meaningful in scale.