To some people, zombies have been overdone. Looking at this brief history of zombie games, its not too hard to see why. But I think the real question is why zombies exploded from standard monster manual enemy to having an entire genre built around them.
Zombies represent the faceless masses. In a similar way that most common enemies in games are alien and wearing a mask. They aren’t human so killing them is more fun in our heads. Now if only we can get zombies to hold the same kind of symbolism that they do in the Romero films, then I think zombie games can take on a whole new way of life.