Wait, you want Marvel villains to start acting like DC ones have since Countdown?
Actually, Marvel villains don't really get along. Look what Magneto did to Red Skull in Acts of Vengeance.
Kang has conquered the Earth before and could have kept it conquered if he didn't have a sense of "honor".
Doom has also already conquered the world before. He gave it back out of boredom.
Magneto doesn't really want the world. He just doesn't want to see mutants persecuted.
Loki wants Asgard. He has no interest in Earth.
The Green Goblin wants to one-up Spider-Man. He doesn't seem to care about the world or anything.
Ultron wants to kill every living (non-machine) creature.
Galactus just wants lunch.
What The Leader wants seems to change based on who is writing him.