Then they'd be alienating the open source community that makes a lot of contributions (though much of chromium is still essentially built internally). They also wouldn't be able to lock down the code that's already been released under the more permissive BSD license.

Now, a fork of Chromium is its own beast. Some searching shows that just to build it takes 30 minutes on a decent workstation. It's huge, which makes me think it's the kind of project that could only really be maintained by a large company. Not necessarily a Google sized company, but a large one nonetheless if you seriously want to remove the dependency on Google.

EDIT: turns out it's Chrome that takes that long to build, which includes things not in Chromium like Widevine, licensed codecs, telemetry, sync, that kind of thing.