Ding ding ding!
The key word is human. Mewtwo isn't human. His mind operates on levels that we can't even comprehend, hell, that maybe even GIYGAS can't comprehend. Giygas is godlike in power, but so are Ganon and Mewtwo.
Also, for the record, I think it's rather unfair that we give Sonic the Chaos Emeralds and no one else cataclysmically powerful objects. If Sonic has to turn into Super Sonic to win, I think it's fair to say that Sonic would lose. By Super Sonic logic, we could just say that any time Ganon was about to lose, he would just wish upon the Triforce that his opponent was dead or in an alternate dimension made entirely of poop.