They literally can't though. Only the owner or a maintainer has permission to merge PRs in to the main branch. Anyone can fork a repo, write a patch, and PR it, but that's where outside contributions stop. It still takes someone from the original team to accept a contribution. Forking a project and then maintaining that fork as a separate project is an option, but it's divisive one that a lot of the community will react badly to if they see it as a hostile takeover or if there's a change in goal (eg making a commercial product out of the abandoned project.)