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
Diversity generates competition that drives innovation. Also, people mostly work on what they like and use what they like. Although it certainly spreads resources on duplicate projects, you can’t force people to work or use your personal favourite. You’ll get used to it.