Are they so different that it’s justified to have so many different distributions?
Linux isn’t a project its a source compatible ecosystem. A parts bin out of which different people assemble different things. The parts being open source means you don’t need anyone’s permission or justification to make something different out of them.
From these many and varied efforts comes life, vitality, interest, intellectual investment. You can’t just take the current things you like best and say well what if we all worked on THOSE when many of them wouldn’t even have existed save for the existence of a vital ecosystem that supported experimentation and differentiation.
If we really believed in only pulling together maybe you would be developing in cobol on your dos workstation.
Flatpak isn’t going to have every library, cli tool, or even every GUI tool. I think in the end out of date just isn’t worth it.