It’s more than just the lack of caves. Bushmen, Andamanese, and other nomadic hunter-gatherers produce hardly any art above ground. They are stuck in the Paleolithic. Remember, these guys can’t even make fire!
There appears to be a lot of work to be done on this subject. It's interesting that at one time in the distant past there was a lot of intermarriage, but that stopped with the advent of the caste system. So, what changed? Why didn't the invaders from the North start a caste system right away?