They were on top for a while, but they've had a massive decline in market share in recent centuries. It's gotten to the point that they've had to resort to building artificial caves!

I think you mean below for awhile...

I'm pretty sure ancient humans didn't live in caves.

It's just that caves preserve their contents pretty well, while the rest of the outside is impermanent.

Caves were often used as burial grounds even in the historic era, so that might contribute too.

After all, why dig a hole for the body when nature provided a ready-made hole already - and, as a bonus, with an eerie atmosphere suitable for last rites.