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
KDE and Gnome are just desktop environments. I use Gnome on Arch which uses pacman as the package manager. I could use KDE or XFCE or i3 or anything. The package manager is tied to the distro, and you can use almost any software with them.