Partly, this is an unwillingness to throw much away.
I would like to think, however, that this is less my laziness, and more because zombies mean different things to different people.
- For some, these creatures are emblematic of a pessimistic world view - the only thing that could unify humanity, is some existential threat like the undead, and there would still be some hold-outs.
- Conversely, its an optimistic thing - in the face of such an impossible supernatural event, people can find shelter, and fight back.
- Its an adventure! Most monsters over match people by far - that is part of what defines them as such. Not much can be done about cthlulu, and a dragon is a tough fight, but hey, zombies you can deal with personally, which is a novel experience.
- Zombies teach the lesson that the most important survival trait is not firearms training, but how to deal with A-holes, and work in a group.
- Zombies are a chance to forget all those social mores about dealing with A-holes and shoot their shambling corpses instead.
- The undead are a symbol of nature and man's inability to truely control the world
- The undead are a symbol of man and its pervasive corruption of the world.