Why do chimps kill chimps from other groups that come into their territory? Why do some chimps use aggression against other chimps to manipulate them, while other chimps use grooming?
A certain degree of sharing is part of our human / animal nature, but so is a certain degree of claiming ownership over things, and certain individuals have more sway over decisions than others. Flat hierarchies with nobody in command seem to work in theory, but in practice it’s different.