If you want to deal with reality, zombies have no circulatory system so there is no way for the food they eat to provide nutrition to the cells. Therefore a zombie would rot away to nothing in just a few weeks.
Blood of Tyrants, I have SOOOOOO MANY physiological and physics questions about zombies that I can’t comprehend why I love the show so much. How can they keep moving on just brain power alone ... ?
I read an interesting article which stated that zombies wouldn’t last two weeks, between birds, insects and bacteria “cleaning up” dead material in the real world.
But of course, this ain’t the real world.
This is true, and was addressed in the Zombie Survival Guide. Wait long enough, and they rot. Of course, until they completely rot, it would be prudent to wear ankle protection, especially since no one is mowing the grass anymore. Never know what you're gonna find in the tall brush...
Plus, it's a TV show. =D