Because internal consistency is not a hallmark of TV programming?
BTW was the cow zombified or was it just dying loudly? Do animals zombify? If not, why not? I think it would kick ass to have a pack of zombie dogs show up, Resident Evil style.
I'm surprised at how much people disliked the most recent episode. I feel like there's been a steady improvement over this season, and that the last couple of episodes are some of the best of the show.