Because if it were, I'd have to declare Pinsir horribly O.P. His face is perfect in place of so many other Pokemon's faces. So many other Pokemon fit perfectly in his face slot. Even his name is a great fusion tool. Sometimes he manages to be horrifying like Zusir.
Or hilarious like Weepinsir and Jynsir.
Or fabulous like Pinkazam:
Or make awesome names like Pinkey, Pinto, and Pinking: