Why don't zombies eventually die in the Walking Dead?

We know the zombies have a fairly simple existence, they can only perform basic functions (like walking) and only care for eating flesh. Yet in the Walking Dead series literally years are passing since the first outbreak and these zombies are still active. Wouldn't their brains eventually die due to low glucose levels or something?

So even if the zombies are still active years later, why so many of them? We know there are other groups in the Walking Dead universe constantly killing lots of zombies every day, at a pretty good rate. Is this ever explained?

Answer

They do

In Letter Hacks #14 and #111 it's stated Zombies eat for impulse and posess no functioning digestive system, with what they've eaten being forced out once its full, unprocessed. It's not required for their sustained animation.

Robert Kirkman wrote on Reddit: ...In the beginning of the show we saw walkers do things like using a rock to help bash the doors in or turning a door knob, is there a reason we've stopped seeing them do that?

"Older zombies are less together and capable or doing things like that. Fresher zombies, which there were more of in season one, are able to do more than older, more rotted zombies.

The worse state of older zombies holds the key; when a human starves, the body will break down fat, then muscle, followed by vital organs. The slide in mental function seems to imply the parts of the brain stem re-animated are canibalized in the process.

Since zombies do not digest, this would be how they survive, and the slower decay that stops them being a bare skeleton in a matter of a few weeks,(#9 and #11) could easily be the virus' way of preserving and stretching its 'food' source.

They would end up running out and 'starving' (in a manner of speaking), but there is no indication of how long, other than that that a surviving human would most likely outlive it, through the years, and that a zombie's lifespan would differ depeding on the individual, (#7); which makes sense, given body fat ratio, muscle density, state of organs and tissue changes from person to person.

Zombies do die off of their own accord, albeight slowly, and it happens whether they eat or not.