I think the easy answer to that is "because it is not as trivial as forking a small app that could run off of a git repo", it's a whole operating system involving a lot of infrastructure and a huge community around it. It might get forked, but people fight probably because they see value in what exists and would rather try and advocate for whatever direction they believe is best. Those who would disagree are not very different, just passive.
An even more trivial alternative is settling for "whatever the founder wants" and seeing the ability to fork as the final justification for this mentality. This is a lot less work, but also can amount to doing nothing, even if shitty decisions are being made. Even if that is your stance, you will have to fight for it. The alternative is everyone just sit idly and pretend not to have opinions. I'd much rather embrace the chaos that comes with collaboration and let it find proper processes to manifest.