A lot of fiction likes to play with the “end of the world” theme. I can’t really say why. Maybe it’s that “even after the world ends, I still love you” feeling that comes off as romantic? Or maybe everyone’s got a little bit of wasteland fever in them. An endless wilderness, nothing left but the male lead and the heroine, wandering the ruins looking for some reason for humanity to keep going. That does sound pretty coool. Use it too many times, though, and it stops being coool.