Any open source maintainer will tell you that many PRs, even well-intentioned ones, end up being a net negative. Often they don't further the maintainers aims, they need detailed review, take sometimes a lot of effort to manage the communication tactfully, etc.

Yes, many PRs are a win and it can be really gratifying seeing your project become bigger and better than you could have made on its own.

But also, many PRs are just a chore and a hassle.