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
The package manager is really only a small part of the story.
A distro at the end of the day is a API/ABI platform. What makes Debian what it is, is that it has a specific set of old unmoving packages. What makes Arch is that it has the latest APIs always. And everything in between like Fedora.
So even if Fedora used dpkg it wouldn’t change anything, you can’t use its packages on Debian.
As to why so many exist… well a lot of them suck in their own unique way.
Except in NixOS, it’s literally a distro built around a package manager. But it doesn’t force you to choose, you can have both unstable and stable packages
Yeah modern usage in general involves silo’d ABIs, be it Flatpak, Nix, Docker/Podman. Modern languages even try to move away from any ABI.
Of course there are upsides and downsides to the traditional approach.