Many zombie narratives, given the nature of their subject matter, take place within apocalyptic or postapocalyptic settings; as the living, sentient humans that populated the globe die and become the undead, the societies they once composed disappear, creating a world without formal governments, religions, or cultures. The possibility of such a post-human epoch on Earth looms large as one of the most salient threats in zombie narratives, along with a fear of being devoured (and that of devouring others) and the terror of losing once's sense of identity while nevertheless remaining (somewhat) alive.
Nick Muntean. “Nuclear Death and Radical Hope in Dawn of the Dead and On the Beach” in Better Off Dead: The Evolution of the Zombie as Post-Human, ed. Deborah Christie and Sarah Juliet Lauro (2011), p. 81.