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
Some of them are more or less historic (for example rpm/dnf and dpkg/apt), where they were developed at a similar time by unrelated projects and have just carried on separately ever since.
Others have been developed to represent very different approaches (such as portage, which is based on the traditional BSD way of managing software by building from source, or snap and flatpak, which containerise applications).
The multitude of systems don’t really cause as many problems as you’d think. As a rule, non-containerised packages need to be custom-built for each distro anyway, so it doesn’t really make any difference which packaging tool is chosen by that distro. That is, you can’t really take a debian .deb package and expect it to work properly on Fedora, even if you install dpkg/apt first.