To me it's not so much that they exist, but more that, aside from Giga Bowser, Master Hand, and it's affiliates, they're not interesting. Galleom's just a generic, vaguely ape looking robot, Duon, while having an interesting design, was and incredibly random and boring fight (attack it until it decides to do something), Galeem and Dharkon did everything right with their final battle, but the characters themselves aren't compelling in any way, and Tabuu, while having a great fight, is incredibly generic in design.