I think that you folks are dramatically underestimating how many people there are in the world at this point.
There are seven billion people on this planet. Its a good bet that the zombie apocalypse only took care of about a quarter of that in the first year.
The thing is this particular ZA comes with the added problem that EVERYONE will become a zombie. Thus even in communities that thought that they were safe and made it through the first year, the could still fall if someone drops dead and they don't expect them to change. That could bring down even well fortified communities and thus create brand new waves of walkers. Hell that National Guard group was from a site that was only recently overrun...more than a year after the initial outbreak.
Add to that survivor communities where folks will inevitably die more slowly due to starvation, disease and tribal warfare...its going to take a while to bring down those billions. The herds that we saw in Atlanta cannot even come close to the actual population of that city. The group we are following could be in a location with a fairly sparse population.
I hate to bring up WWZ, but that book painted pretty grim but elaborate picture of the difficulty in reclaming the planet after a ZA. It took 10 years to win the war....and it took that long in a ZA where everyone was not infected and thus were not guaranteed to turn.