Why is it incumbent on the developers (who know their codebase and its capabilities better than the forum posters) to explain their decision to the forum posters begging for a feature, and not only explain it, but explain the decision and the reasoning behind in such clarity and depth that the forum posters (who might not even be programmers, let alone know the code base) understand it sufficiently well to refrain from begging and insulting?
Because - and more so for the maintainers/managers of those projects - being able to communicate effectively is part of what it means to run a successful project?
That may even mean that you have to hold yourself to a higher standard than some low-effort post on the part of some casual user. Especially if you're the boss, the maintainer, the leader. And if that's asymmetric then yes it is - but to be a maintainer/manager of a project is also asymmetric. Good managers might also promote that culture more widely across all the project's contributors and so yes it may apply to all developers, too.
Not everyone that engages with your project is going to be perfect, some may even be rude. But as a representative of that project it's a skill to be able to cope with that, on behalf of the project (not you). I think one of the most underrated skills of a FOSS maintainer is a degree of fault-tolerance, to use a systems analogy.
Or, you could argue that no, it's not incumbent on a maintainer to be anything, even to be kind. But then don't expect your users to come back.
