Zombie population grows exponentially. One infected undead bites someone, creating a new zombie. Much like that old shampoo commercial, they each bite two friends and so on and so on and so on. But humans have to have sex and give birth and grow from infant to adulthood. And they generally can only kill one zombie at a time. It takes time and effort and resources to build up to a plan and an opportunity to get rid of a whole bunch at a time.
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While the zombies may be able to propogate and multiply faster than humans, the humans probably will win in the end, because they have everything to lose.