And really, there's nothing saying your idea hasn't happened. It just hasn't happened where the characters are. To address JD
's post above, leaving it unclear what's going on in the rest of the world allows for that possibility (including the possibility of rescue).
If you actually come out and state (or show) that the entire world is overrun, you're unnecessarily restricting the storytelling options. Right now, it's still possible that the show will wrap up by having the characters escape to the Colorado Safe Zone (or whatever) and live happily ever after.
Well, at least until they die by other means, when a head-shot will still be necessary.