It is just one of those inconsistencies that you have to you have to ignore in most zombie films, as there is no good reason why zombies would be that much of a threat. Its the reason why films like 28 Days Later and WWZ changed certain aspects about zombies, to make them a more credible danger. I personally like how Shaun of the Dead did it, the zombies were dangerous to a bunch of unarmed civilians but once the army shows up at the end they just wipe them out effortlessly.

Some points as to why traditional zombies like those in the The Walking Dead are not really a threat. 

1. The traditional method of spreading the disease is direct exposure of your blood to the infection, which is probably the worst possible disease vector imaginable, it just wouldn't spread fast enough (see for example why the human race hasn't been wiped out by Rabies). I know in the series it follows the line that everyone is infected already and its only when they die that they become a zombie but even then it would have been fairly simple to control by incinerating the dead or just shooting them in the head once the person has died.

2. Zombies are slow, making them easy targets for anyone with a gun, during battles in WW1 a lone machine gun nest could kill literally thousands of people in one day, and this was against armed soldiers, now imagine a zombie scenario with modern weapons, it would be like shooting fish in a barrel.

3. Zombies are stupid, they wander aimlessly about, the amount of them that would incapacitate themselves through accidents or falling into traps in the environment (holes, ditches etc) would be huge. They also head directly for the nearest sound or visible person which basically makes them dumb sheep, they would be fairly easy to herd into traps or areas that could be sealed off.

4. Vehicles, cars are very powerful machines and could easily plough through a crowd of zombies with ease and not get bogged down. This is even more true for larger vehicles like trucks, who's drivers could happily go for hours squashing zombies, one guy could easily take out thousands this way.

5. Weapons, most military grade weapons would knock a zombie on it arse even if they didn't kill it head and as you move up the chain things like heavy machine guns would just blast a zombie to pieces, it doesn't matter if you are not hitting it in the head when you leave it in 5 separate pieces.  And things like explosives would be even more effective since zombies lack of intelligence means they could easily be herded in tight groups and blown up.

6 Longevity, under most conditions a normal body would be in pieces after 6 months let alone able to move about. In fact moving about would only increase they rate of damage to the dead body, so most zombies would be completely immobilised within months. And if we assume that they decay slower than normal there is still no explanation as to how they have the energy to move about, they can't be getting it from eating as they are probably the worst predators one could possibly imagine.