[BreakABone]

Okay let's think about it this way

There are only so many materials for our robotic overlords to use before they can no longer produce more.

Than can be an endless supply of humans.

But we have effective means of killing zombies with blows to the head, its been really difficult to kill the robots as I've seen.

So one has an endless wave, but easily killable... while the other has a finite supply amount, but no means to kill them effectively.