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 package managers do have differences that justify a separate project (nix, gentoo’s portage, etc).
For others, sometimes package managers are very similar feature-wise. But some developers would rather remake the thing because they would understand their code a lot better than someone else’s. Or because it would be far easier for them to customize rather than extend another project.
Imo it is developer laziness. Being able to use other people’s work is a valuable skill. But then again, this is open source, and people are free to develop the software they want the way they want.