Much of our hobby is spent creating or exploring and defiling dank,dark, dangerous places that have never and will never see the light of day. The player characters go there because that's where the treasure is. The creatures they take the treasure from live there because that's where the DM said they do, generally because a book told him/her so.
But why do these places exist in the first place, especially in such huge freaking numbers? Creating underground spaces is hard, dirty dangerous work. Excavation is no joke, requiring skills and experience, a massive expenditure of time and effort (and probably lives). Building above ground is quicker, cheaper, and easier.
It's easy enough to explain why one microdungeon exists in the world (tomb, modified natural cavern, etc.) but most adventurers will delve into dozens of downbelows if their careers are even middling long. so without further ado, here are a few global and regional reasons why your campaign has so many holes in the ground stuffed with monsters and treasure:
1.
Sunspots Every few centuries/millenia, the sun has a massive case of heartburn, bad enough to lightly toast the surface of the world.
2.
Plague of Locusts But these locusts are the size of elephants and number in the millions. Nobody knows where they come from, or when they'll be back.
3.
Geomantic Empire In ages past, an empire arose with stunningly powerful earth magic. Excavation was child's play to them. Also the pinnacle of beauty for them was fishbelly white skin (hey, if
bound feet was a real thing...).
4.
Because the Night.... Belonged to the Hunters. Extra-dimensional beasts of godlike powers, they harvested sentient species in the millions- but only above ground, and only at night. Nobody knows why, they're all just glad they've been gone for generations and hope like hell they never come back.
5.
And the Gods Said "For each House ye place Above, so must ye build Below; for ye were raised from the Dust, and to Dust ye must go."
6.
The All-Seeing Eye should really be renamed, because people figured out pretty quickly that it couldn't see through more than about six feet of dirt.