Shouldn't many if not most zombies in The Walking Dead have their guts spilling out and the majority of their tissue eaten? Do zombies have a maximum capacity for food - after which they'd rather give the victim a scrape rather than a gorging?
In The Walking Dead, zombies do not attack or eat other zombies. In season 1, we see this explicitly... two protagonists manage to escape by covering themselves in so much gore that the zombies do not perceive them to be alive.
Thus, any victim of a zombie attack quickly falls into two categories: they're killed quickly or they're not killed quickly.
If the former, they almost immediately reanimate as a zombie. They're no longer subject to attack by other zombies.
If the latter, they're most likely still mobile and defensive, and thus able to evade further injuries (short term, anyway). They may crawl off and die later, or maybe even survive (I stopped watching at the end of season two... it's fuzzy for me if they survive bites and such).
In this way, almost everyone should eventually become a zombie. Only the most devastating attacks both kill their victims quickly and dismember them so completely that they will not become a zombie themselves.
I think your presumption that being a zombie == being eaten/bitten at some point is wrong.
It is TWD cannon that EVERYONE is infected by the virus, they just haven't died in order to be brought back by the virus yet. I don't know what the initial infection vector was, but for a lot of people it wasn't lethal. Only when they die from another source do those people turn into walkers, either by suicide, another infection, accident, what have you. So it's no surprise that a decent amount of the walkers have little to no physical damage to them.
If the Walking Dead is like most Romano Zombies, stomach capacity is not a limitation. They will eat and eat even if the stomach is full. The only thing that stops them is the end of normal brain activity in the victim. Once the normal living cerebellum activity stops, and the Zombie virus takes over, the zombies will stop their feasting. As death via trauma and blood loss can be fairly instant, that means the second the victim dies, the zombies will stop eating, hence the fairly intact zombies.
For example, death by Exsanguination takes between only 14 and 70 seconds.
There are probably cases where the person died naturally or via suicide. The zombies may not be able to reach a body or don't eat the whole thing. Although, yes the numbers should have decreased hugely by now, how else are they supposed to do a series if the antagonists are all dead and there are no longer any living dead.