Maybe zombies are simply evil and like the taste of human flesh.
Anyway, walking dead isn't about the zombies but about the survivors. It's a psychological story about how fear of zombies basically drives civilization to collapse. Bandits do more harm than people - as is shown, they have weapons, and their evil lets them whipe out the defenseless and each other that much faster.
Zombies on the other hand? Are slow, and don't reproduce. If they do (life to unlife transformation) they're so stupid that they might eat one of their own in the process. There's no way nature intended to allow them to continue living - unless, they can manage to evolve in the next decade or so before they all become extinct due to isolation from proper prey.
The final explanation is a rather... a different take altogether. The disease that turns someone zombie is a very complex mutation - rather than genetically changing the person somehow (what disease/abnormality actually does that, unless you were born with it?), it lends intensely psychotic insanity.
The body is fueled by adrenaline (it never rests), 'forgets' all pain, and basically just continues. Effectively, it'd require a lot of energy to keep going. Somehow, the mutation allows the body to produce a lot of adrenaline and forget all else.
Essentially, the fear mechanism takes hold and the social detectors common in the brains of all humans is rewired - the zombie recognizes other zombies and is 'afraid' of people (as much as a mostly brainless automaton of decaying flesh can be); it's fight or flight reaction is to always fight and to eat and flight is written out of the equation. It sniffs not for the decaying smell, but the pheromones of humans. So the sexual chemistry of a zombie is probably rewired as well. Certain basic instincts remain, such as audio comprehension - so it responds to loud noises.
Well, that's my explanation. Come up with another one, if you like.
.......I would love to see a modern day zombie show that ended with a crazed necromancer wizard who had been around since 0A.D trying to perfect his craft.
"Oh yeah... no. Who me? Yeah... no. I'm a wizard, though. Yep. Been uh... been engineering the zombies for X years now. Yes, I worked in Y office down the street, that's not the point! The point is... now, here's the point. You're all going to die, because uh... because, I'm gonna engineer an even worse virus for you meddlin' kids - *is shot*"