Considering how much time he spends picking fights with Crazy Dave, the Zombies should not be that far along on their organized quest to conquer the world.
He spent an entire game chasing after Crazy Dave through time trying to stop him from eating a Taco, when he could have spent it more productively, like conquering the world, or at least neighborhood, in Crazy Dave and the Player's absence. Or, with time travel, conquer time periods that Crazy Dave and the Player did not visit, to ensure that zombies came out on top in history.
Considering how far along the Zombies are, despite Dr Zomboss's singleminded feud with one (two if you count the Player) person, someone bigger has to be pulling the strings.
