Zombies are supposed to be dead but still walking around. The horror is, among other things, an existential one: that our dead are coming for us, relentless in their pursuit to reduce us to their condition. We see these shambling, decaying bodies, and are reminded: on some level, that’s us. Inevitably. No matter how hard and how long we fight. A zombie is a particularly hideous memento mori.
Of course, you can take that basic horror and ramify it with specific social/cultural/political points, as Romero and others have done, but you build on that base of existential horror.
The modern “viral” zombie is almost a complete inversion of this. The group doesn’t inevitably and hopelessly succumb; instead, there’s the excitement of watching them (some of them) defeat and escape from the plague. Modern zombie movies are fantasies of election – we identify with the survivors, and so become convinced that we would be like them. And these survivors seem so much more alive than we do – partly because they’ve escaped the constraints of civilization, but not only because of that. Since zombies are death, that fantasy of election is a fantasy of escaping death entirely – precisely the opposite of what a zombie is supposed to make us feel.