It's certainly not because zombies are so badass - face it, zombies are the lamest adversaries ever, as I blogged about a year ago.
It's recently come to my attention that Cracked beat me to the punch by several months: 7 Scientific Reasons a Zombie Outbreak Would Fail (Quickly)
I pretty much covered their reason #1 in my post on the topic, but they cover it at greater length and more entertainingly:
As we touched on briefly above, if Homo sapiens are good at one thing, it's killing other things. We're so good at it that we've made entire other species cease to exist without even trying. Add to the mix the sheer number of armed rednecks and hunters out there, and the zombies don't even stand a chance. There were over 14 million people hunting with a license in the U.S. in 2004. At a minimum, that's like an armed force the size of the great Los Angeles area.
Remember, the whole reason hunting licenses exist is to limit the number of animals you're allowed to kill, because if you just declared free reign for everybody with a gun, everything in the forest would be dead by sundown. Even the trees would be mounted proudly above the late-arriving hunter's mantles. It's safe to assume that when the game changes from "three deer" to "all the rotting dead people trying to eat us," there will be no shortage of volunteers.
Plus, if we look at zombies as a species, they are pretty much designed for failure. Their main form of reproduction is also their only source of food and their top predator. If they want to eat or reproduce, they have to go toe to toe with their number one predator every single time. That's like having to fight a lion every time you to want to have sex or make a sandwich. Actually, it's worse than that: Most top predators are only armed with teeth and claws, meaning they have to put themselves in harm's way to score a kill. Humans have rifles.
More fun articles from Cracked (warning - it's a time sink)
6 most insane people ever to run for office
The CIA's 5 most mind-blowing experiments with LSD
6 Famous Geniuses You Didn't Know Were Perverts
I don't know if I exactly agree with the label "pervert" in some cases... but warning - you may have a hard time forgetting the image of Rousseau accosting women with his... well, anyways, you have been warned.