Post-Apocalyptic fiction is sci fi like Godzilla is sci fi.
At its heart, sci-fi is about "what if?" Post-apocalyptic fiction answers a rather important "what if?" question about how individuals would or should react to the collapse of society. The concept is no more and no less sci-fi than anything done in space.
Technically true, as I said, just as Godzilla answers "what if atomic testing woke up a big-ass monster." But when people go to see a Godzilla movie, they go to see big-ass monsters, and when they tune in to Walking Dead
, they tune in to see Zombies. Nobody is saying, "Gee, 2001
rocked, I think I'll check out Walking Dead