Are they so different that it's justified to have so many different distributions? So far I guess that different package manager are the reason that divides the linux community. One may be on KDE and one on GNOME but they can use each other's packages but usually you are bound to one manager
Incorrect. There is manjaro, but there also is msys2, a windows program with the goal of making linux tools available on windows by recompiling all of them. That's very far from the arch philosophy and repos.
And ubuntu and debian have massively different repositories. One of them gives you the actual firefox package, and the other installs firefox via a closed source backend, app store called snap, when you attempt to install firefox using apt.
And then there is also the version differences, like debian stable is going to have much older software than ubuntu.
Thanks for pulling corner cases from dark places... not sure if we misunderstand but my point was as written, you use the package manager/repository which ships with your distro. So the original quote was: