If you caves spawned in explored areas your would be even worse off. At least this way you know all the places you don't need to search. I would suggest that very few people have explored the entire world.

For me it took some searching but the once cave I have found was up high but still on the edge of the mountain biome.

To be honest if you have top tier gear and have found the trader, which you obviously have having explored the entire planet, then the caves are going to be a little underwhelming as they are design to fit into a toon that is not yet ready to enter the plains

We have started a new game to get a better sense of how they feel when you go in from a fresh start.