While I do agree that seeing raw manpower invested in so many distros could look like a waste, I think that motivation is the fuel that make us move forward. Some people create new distros to learn or experiment new ideas that do not fit existing distros (NixOS, Gentoo, Arch) others because they want to experiment, starting from a solid base (all the based on debian/ubuntu/fedora) or because existing distro do not allow implementation of those ideas, for lifecycle or because the community vote against that idea (you can propose, but have to accept also no as an answer).
In the end each one is trying to solve an issue that has not found home in the already existing distro, or simply, is trying to scratch an itch. Is there duplication of efforts? Probably yes, but that effort lead those people in learning and become better. Asking or forcing people not to do what they feel, will lead them away from Linux in toto. Sometimes feels good to create something and being a king of the rock rather than being just another anonymous contributor struggling to find space for your ideas.
And that is also a challenge for the main distros, to welcome and being open to new ideas in order to be place where innovation can happen. Otherwise people will go elsewhere and create their own distros.