[jlokier]

> I fail to see why it matters that your PR isn't accepted.

Making their code useful to other people is a strong motivator for many altruistic open source developers.

The purpose of getting PRs accepted upstream is so that your changes are useful in concert with other people's changes, especially their later changes.

Your private change is of little use to others when it's only in your private fork, which hardly anyone knows about, and if they do know, your change is incompatible with later work and maintenance by other people anyway.

If you don't care whether anyone else uses your work, especially in future, that's fine. But if you want it to be more useful to others, getting it merged upstream makes a huge difference to that.

(Merging upstream or just submitting PRs that don't get merged also helps personal visibility and reputation, which is undoubtedly a motivator for many. That's a burden on upstream maintainers when someone is pushing low quality work and doesn't care to ensure it's useful to the project.)