Because mostly if you are in caves there are bats (although, I ever gone to a cave, but there weren't any animals there, but that's probably because many humans come there, and there is a place that you ain't allowed to go for a reason).
And for two: Zubat isn't in the Black and White series. And like sumwun stated, not in these caves too. (I'm just trolling)
Zubat is supposed to be a Pokémon to irritate people while in a cave, to make it feel more like a challenge, with moves like Mean Look. (And probably Confusion, not sure, although, I think so) It's like Tentacool/Magikarp in the sea, Zubat in the cave.