I kind of wonder if this has something to do with particularly American conceptions of frontier and Manifest Destiny. Having reached the coast and filled it up with people and development, do we need to depopulate the continent in order to feel like our main characters can explore, and discover things, and have adventures?
I think it’s actually a good deal simpler than that. For you to have a grand Manifest Destiny narrative, you’ve got to have protagonists who have an interest in making new discoveries, conquering new lands, and so on. In most of these stories, on the other hand, people are just struggling to survive in a hostile, unfamiliar world. That leads me to think that the modern obsession with zombies, the collapse of civilization, and so on, has far more to do with our own anxieties about decline—particularly in the United States. We’re transfixed by post-apocalyptic scenarios because it so often feels like we’re sliding towards an apocalypse.