Whoever contributes to it controls Chromium.

If 90% of the contributors were non-Google, then it would effectively be controlled by non-Google, because they could fork it and their fork would still get 90% of development.

See Terraform for a live example.

The only reason Google "controls" Chromium is because Google funds almost, but not quite, 100% of its development.

On a similar note, there's nothing stopping Microsoft from investing equal or greater amounts and forking Chromium (well, arguably they might already have with Edge). Except that they're benefiting from all of Google's investment, for free. Why turn down a massive developer investment from your competitor with no strings attached?