The popularity of the zombie horror genre has skyrocketed in recent years. Why? I believe that it's so huge because there's a sense of Uncanny Valley. They look human enough, but they aren't quite there anymore and it's super creepy. Or maybe it's some sort of wish fullfillment. If a major apololypse does happen, we like to think that we'll be among the survivors. The survivors in most zombie movies are so stupid that it's a wonder they survived that long. The audience recognizes this and thinks "Oh, well I'm smarter than that so I could survive too!"
Or maybe it's just that we all just love to watch a good old shoot 'em up.