The real reason zombie movies exist is to fill a plot hole in the usual post-apocalyptic survivalist story. The way that story usually goes, a big catastrophe happens, civilization as we know it is overturned and everyone has to wander or build their own mini-societies, recapturing the hunter-gatherer lifestyle, in response to the disaster. But the plot hole in that is always, why don’t people just set up refugee camps and try to rebuild civilization.
The survivalist story usually assumes that civilization is rotten underneath, that it only takes a push before people are killing each other over tins of canned food. You can find the prototype of the “zombie free” zombie movie in Heinlein’s Survivalist essays about a nuclear war
“If the fragile structure of that city were disrupted by a single atomic bomb, those who survived the blast would in a few short days be reduced to a starving, thirst – crazed mob, ready for murder and cannibalism.”
That’s a few days in Los Angeles and there are your zombies. (This also gives Farnham’s Freehold some context, Heinlein thought that after a nuclear war everyone regardless of race would go cannibal in LA.) But this doesn’t pass the common sense test. If you made a movie where everyone went cannibal a few days after a nuclear bomb, it would bomb. Because we don’t believe civilization is that fragile.
The Survivalist story assumes most institutions and people are rotten underneath, that the only thing keeping us from beating each other to death all the time are cell phones and security cameras. Take them away and it’s zombieland. But most people don’t buy into that.
The zombie movie fixes the survivalist plothole. It usually dumps the nuclear apocalypse, and cuts right to the starving mob ready for cannibalism. That’s what the zombie is. The average citizen after a disaster who can’t take care of himself as well as our heroes becomes a zombie. (S.M. Stirling actually plays that out in his Emberverse novels where the people without skills turn to cannibalism and then turn into crazed mindless zombies.)
A convenient zombie virus, lets the survivalist narrative play out, hitting the reset button on civilization, without challenging the audience’s faith in humanity. The zombies are what survivalists think of most ordinary people. The living dead, who didn’t bother getting ready for the end of the world.