There are no free lunches, pull requests are no exception. For starters, before merging every pull request needs to be reviewed at a minimum. That by itself can

There are no free lunches, pull requests are no exception. For starters, before merging every pull request needs to be reviewed at a minimum. That by itself can oftentimes be a very time-consuming activity, especially if the changes are from someone outside the circle of regular contributors. Outside of fixes for typos and other trivialities, pull requests generally require a lot of back and forth to get to a good state — does this change make sense architecturally, does it cover edge cases, does it come with tests? Additionally, oftentimes pull requests expand the scope of what you need to maintain, whether you want to take on that permanent burden is a critical question in and of itself. The list goes on. There are many projects that do make it work, but make no mistake that this takes a considerable amount of effort.