[munificent]

You're just restating the parent comment's point.

Assume that prehistoric people drew pictures all over the place, which, given the inclinations of modern humans seems to be true. Then assume that every easily accessible place is likely to have its art erased over overwritten by later humans. Also a reasonable assumption.

What's left? The only ancient cave art we find is the stuff that's highly inaccessible.

In other words, the cave art we see today doesn't reflect the behavior of the original artists as much as it reflects the behavior of later humans that would intercede.