[WalterBright]

They never starve to death, or even weaken. They're not impaired by not having a stomach or a circulatory system to provide energy.

The dullards who fight them never think of putting them in a hamster wheel and generating perpetual electric power.

Besides, the zombie armies never seem to have any logistics to keep them supplied with brains, or any support logistics at all. There's a reason why predators never evolved armies.

(Ok, army ants are predators. But they evolved logistics!)