Let's say that we're going to make an alien invasion film. The film starts with a dramatic meteor storm that results in space rocks falling to earth all around the globe. These rocks (in an obvious ripoff of The Blob) contain spores that grow, over the course of a few weeks, into person-size alien creatures. The aliens will have a horrific appearance: some sort of rubbery, glistening, half-insect, half-lizard nightmare. Not even remotely human. They pop up in the millions all over the planet, and begin seeking human flesh to feast upon.
Sounds pretty scary, right?
But they're slow. They're so slow that they can't catch you if you're moving at a brisk walk. They're pretty strong, but they don't have any poison stingers or laser guns or super-sharp claws. They just have to hope to catch you by surprise or surround you with superior numbers, grab hold of you and gnaw you to death.
Also, they're kind of stupid. They'll blindly wander into traps or stagger forward into oncoming gunfire, without any sense of self-preservation. They are kind of tough to kill, but the people of Earth quickly learn that a single sharp blow to a particular sensitive location will kill them instantly.
Not so scary any more, right? In fact, this sounds like a pretty boring movie all of a sudden. The so-called "action" scenes would just be a bunch of CGI alien critters lumbering slowly forward in a straight line while the heroes calmly pick them off one by one.
But if you take out the slow, stupid aliens, and put in zombies, suddenly you have the same concept that's been driving countless movies, video games, comic books, and so forth, for decades.
So what's the big deal? Why is the basic concept behind the original Night of the Living Dead movie so popular that we keep seeing the same zombie-apocalypse idea come out over and over again? What, exactly, is the appeal?
Some properties (such as The Walking Dead) try to use the basic idea of a global catastrophe to tell more compelling, human stories. Sometimes we'll see a variation on the basic zombie characteristics (like in 28 Days Later). but it's safe to say that the vast majority of zombie-related entertainment is about a hero or heroes who, when the apocalypse comes, go round up a pile of guns and blast their way to safety through hundreds or thousands of mindless, slow-moving, animated corpses.
I have to wonder if most zombie entertainment is so popular because they are, essentially, mass-murder fantasies. The heroes in these stories are able to kill vast numbers of people with impunity. This mass slaughter is justified because (1) their targets are supposedly already dead, (2) the zombies are, in their slow, methodical fashion, trying to kill the heroes, and (3) society has collapsed, so there's no one to really pass judgment on the heroes. The story allows the viewer (or reader, or player) to temporarily live in a world where it's okay to kill as many people as they like (as long as those people are "zombies"). If the targets weren't human, the appeal of the fantasy is gone.
Just my low-budget pop psychology rambling for the day.