On a metaphysical level zombies touch that inner core of our fear of our own mortality and what occurs after we die. The thought of a person walking around that is nothing more than an empty shell - human being sans soul, if you will - who only craves the flesh of the living (as you'll notice zombies never gnaw on each other) touches a raw nerve in many. Some recoil from that fear; some accept it; others embrace it.
And then of course there is always the question of does that walking corpse of your loved one still contain a part of what they once were? Do the shambling, rotting undead still hold a spark of intelligence? Do they still posses the capacity for human emotions? If we are to strip away the "soul", the spark that gives us the capacity for rational thought and reasoning, are we really nothing more than mindless creatures, resorting only to our most basic needs, i.e. food?
It's also a metaphor how we, as humans, devour each other mindlessly as we progress through our lives in order to attain our goals, which is basically the satiation of our desires; or hungers, to put it another way.
But mostly, zombies just rock!