Perhaps it is just that zombies are the only safe enemies to have, now that referring to real people may annoy (a) people to whom you owe a lot of money; (b) people on whose economies or natural resources you depend; (c)people who can bomb your property and kill your nationals; (d) people who may or may not have nuclear weapons and itchy trigger fingers; (e) people whose votes you want; (f) any of the above.