Let's pretend for a moment that zombies are real (as if half of you weren't already daydreaming about that very thing). Have you noticed how most zombie movies take place only after the apocalypse is in full-swing? By the time we join our survivors, the military and government are already wiped out, and none of the streets are safe.
There's a reason the movie starts there, and not earlier. It's because the early part, where we go from one zombie to millions, doesn't make any sense. If you let the creeping buzzkill of logic into the zombie party, you realize the zombies would all be re-dead long before you even got a chance to fire up that chainsaw motorcycle you've been working on. Why?
7They Have Too Many Natural Predators
Do you know why we, as humans, are at the top of the current food chain? Not because we're hard to kill (well, with the exception of Steven Seagal). We're not; we're little more than tasty flesh bags waiting for an errant horn or claw to spill our guts like a meat pinata. No, we're on top simply because we are so absurdly good at killing things ourselves. A good offense, as they say, is the best de-LOOK THERE'S A DUCK! MURDER IT!
We are simply too smart and too well-armed for any wild animal to hunt. Now consider the poor zombie. It lacks every single advantage that has kept humanity from being eaten to extinction. It wanders around in the open, it can't use weapons, it can't think or use strategy. It doesn't even have the sense of self preservation to run and hide when it's in danger. And, it's made entirely out of food. It's easy prey for any animal that wants it.
If you're saying, "Sure, but it's not like my city is full of bears that can come eat all the zombies," you need to think smaller. Insects are a major pain in the ass for living humans, and in some cases, being able to swat away flies and having an immune system is the only thing keeping us from having our eyes and tongues eaten out by maggots. Zombies in any part of the world with a fly problem are going to be swarming with maggots in short order, meaning that most of their soft tissues will be infested, and their eyes will be very quickly useless.
Not so disgusting now, are they? OK, yeah, but show a little respect.
We'll scale up a bit: In America alone, we have bears, wolves, coyotes and cougars, all of which can put well-armed, thinking, fast-moving humans on the menu, if the conditions are right. To most predators, the "right conditions" are when the animal is weak or infirm, or otherwise generally unable to defend themselves, like a walking corpse. Hell, just think of the millions of stray dogs out there who'll quickly learn that zombies are an easy meal.
Now imagine zombie hordes wandering Africa. Between lions and cape buffalo (and hippos, and rhinos, and elephants), we'd finally have a disease that Africa is better suited than the rest of the world to defend itself against.
6They Can't Take the Heat
It's generally accepted by zombie experts that they're going to continue to rot, even as they shamble around the streets. What the movies fail to convey, however, is the gruesome yet strangely hilarious effect the hot sun has on a rotting corpse.
The first concern is putrefaction. Thanks to the plethora of bacteria we use in our colon for digesting plant matter, called gut flora, our bodies are ripe for decay the second our heart stops. Since heat speeds the growth of bacteria (which are plenty happy to start feasting on you once your immune system is no longer a concern) the zombie's got a looming expiration date the very second it turns.
Dead bodies bloat because of the gases created by the bacteria, meaning that in warmer areas even Abercrombie Zombies are going to start getting fat in the first few days. After a few weeks of this, the nasty, bloated zombie army is going to start doing something that is simultaneously the most awesome and disturbing thing a zombie can do: they will start exploding (CAUTION! Pictures!). The warm, moist conditions in the tropical and subtropical parts of the world (or even just summer in the temperate parts) speeds this condition, meaning a July zombie outbreak pretty much anywhere would be over in a few weeks just by virtue of the rampaging monsters bursting like rancid meat balloons.
At the other end of the heat spectrum is dry heat. If you're in Phoenix or the Sahara when the apocalypse hits, the zombies might begin to mummify in the blazing sun and heat. While the normal symptoms of dehydration are not a concern for a zombie, there is the problem of desiccation. With no reasonable means of replenishing the water in their cells, zombies walking around in the Texas heat all day are going to suffer cell damage due to direct sun exposure to their skin, and thanks to the drying effect wind has, the Southwestern dead will stumble around more and more ineffectively until, at some point, they simply drop and wait for the scavengers to come pick them up for the annual Slim Jim harvest.
So they'd better hope the outbreak happens during the winter, right? Well...