The Linux kernel is actually a perfect example of this.
It’s worked on by hundreds of companies, and the bulk of the work is done by a small number of megacorps.
If it was worked on by a group of volunteers doing bits whenever they had spare time, it’d be in a much less useful state right now.
You’re seriously underestimating how large and complex web engines are. There’s a reason we’re down to 3 engines and the community hasn’t been able to create one.
It’s hard to do. It requires hundreds of millions a year to keep going.
If it were genuinely so trivial to maintain a browser engine, more would be doing it. Even easier, Firefox forks could take over maintaining the engine, as opposed to just tweaking the browser (not even having to work from scratch with a new engine). But they don’t, for the reasons I’ve already mentioned.