Because they don't go to the mailing list. For a lot of OSS communities, if it didn't happen on the list, it didn't happen. That's certainly how it is in all the big projects I've interacted with. I'd LOVE for a way to cc a mailing list on all PRs to a project, but that's not possible AFAIK.
Most PRs I reject with "this should be a patch sent to the list" simply because I'm not always the only one that should review something.
> For a lot of OSS communities, if it didn't happen on the list, it didn't happen.
Yeah, well, maybe that's a bad idea?
There is another big FOSS project I know which has its own bug tracker from way back when (horribly outdated, frequently malfunctioning, terrible usability) and then they have their new GitHub tracker. The result? If you want to file anything, you have to file it in both systems.
What I'm trying to say is: You have to make a choice at some point. Either you move forward and solve problems, or you stay where you are. But don't blame the move forward for not allowing you to stay where you are.
Most PRs I reject with "this should be a patch sent to the list" simply because I'm not always the only one that should review something.