Just like with any other OSS project, you have to convince the upstream developers that your PR is worth merging.

You don't do that by throwing PR's over the wall and then moving on. You do that by being part of the community.

That said sometimes you just don't have the resources to engage another community at the moment, so you push the PR over the wall anyway, assume it won't ever land and act accordingly.

The smaller and less impactful the change, the bigger chance of it landing. I'm always clear with my PR's that I push over the wall though: I probably won't be around to maintain this, feel free to not merge, etc. I also try to thank them for their service to the community and share how their code made my life easier.