There is no real outer selection pressure like from a predator so nature can go full retard here. Just like with human.
Except that humankind had to deal with shitloads of predators, which made us develop so quickly in the first place. And human is a wonderful creacture, and the best proof is that someone of our kind invented computers, then someone invented internet, and then chadz made cRPG. Beat that, kangaroos.