who should do packaging for each distro? - upstream maintainer: too much work. each distro requires certain best practices/convention. - distro: may not meet certain standard set by upstream maintainer.
Whoever's going to do the best job. Sometimes that's the distro packager, sometimes it's the upstream app developer. With complex applications with lots of dependencies and a large surface area for breakage between versions of dependencies, it's far more likely that the upstream developer will be in a position to do a good job than a distro packager who's responsible for 100 other packages, and maybe never actually uses the application, especially when they're constrained by distro policy on vendored dependencies or multiple library versions (which, like, I get the reason for, it sucks for a security fix to need applying in many different places, but it's a policy that will inevitably create bugs).
This is something which grates when reading the defense of Fedora's flatpak repo in the thread linked elsewhere in the comments, claiming that Fedora is going to package with much higher quality and more testing. I can believe that relative to some rando 3rd party on the internet, maybe (at least that rando probably has packaged it because they would like to use it personally), but relative to the developers themselves? I think that's pretty unlikely.