Another theory stated that humans are already dead by now but not maybe of the nuclear weapon, maybe it's by the monsters after the events of the nuclear event that have changed the earth. The monsters probably killed off the humans  through out the years, or maybe it's the new adaptation to the earth since the event, the humans have no weapons or medicine to protect themselves from a whole new world  such as, Monsters, Dragons, Diseases, Even the climate change

That's all of theory#2