Because it's bad tv (plus hard to film) if it's just a pile of dead bodies everywhere. Half a million people in Atlanta, goes to a couple million if you count nearby outlying areas. Obviously we didn't see a couple million zombies or corpses. We also saw about 20 survivors, so feel free to extend those numbers out as far as you like. If there's even 1000 zombies seen for every 20 survivors, you're still looking at .001% survival rate. (projects to 70k humans if you use worldwide numbers, but some areas would be much less impacted, and wouldn't see such bad numbers.)
Atlanta urban area is like 5 million people, not half a million.
We've seen a few different groups mind you. The old folks home from season one, the gang from season two, Hershel's farm, now the town, the prisoners, the group from the latest episode, etc. That adds up to a lot more than 20 survivors. While it is clearly a small fraction of the overall population, if you extrapolate it out to other areas and assume even a few neighborhoods or facilities that managed to survive the initial outbreak and shore up their defenses, there could be areas with tens of thousands fairly easily.
I think it is safe to assume at least a few million people are still alive at the time of the show's most recent events. Is that enough to keep the hordes replentished? Of course not, but that assumes they need replentishing. Once the dead are walking I don't see any reason they should be dying off out of something as simple as starvation or exposure.