Question about zombies in films, TV shows, etc.?

Ok, so the idea of zombies is pretty well known. You get bitten, you turn, you start biting others, and so on and so on. Question though, if when a zombie catches a person they begin to eat them, wouldn't they, and other zombies nearby, just keep eating until nothing was left? How are there always so many zombies that they completely overrun Earth? And they generally all look like their whole body is intact. Wouldn't most of them be so messed up from being eaten that they couldn't walk around and attack others?

The Walking Dead explains this a bit better by having the "rules" be that dying in any way causes you to come back as a zombie (or "walker). In that case, it makes sense that there are so many zombies. But in other films, how are there always so many? The only way I can see a person whole enough to actually be a threat when they turn is if they were just bitten, then got away very quickly and wasn't put down before turning.

So what do you think? Does it make sense that there are always thousands upon thousands of zombies that seem to have simply been bitten once and then left alone until they turned?