Why There Are So Many Dungeons
While I like the old idea of living dungeons, those would be very rare—even singular.
The mythic underworld seems more usable, but its natural entrances at the surface would likewise be very rare. “Man”-made dungeons that delve sufficiently deep could become supplemental entrances.
But why would so many sentient beings construct so many “mundane” dungeons? Especially when those dungeons are unneeded, excessive, and weirdly laid out. Because dungeon building is an insanity that can arise in anyone who builds and inhabits a sufficiently large, permanent structure of any kind.
So aristocratic castles; wizard towers; religious temples; dwarven homesteads; humanoid cave towns. Anything.
Where does the insanity come from? Obviously, it is the mythic underworld reaching out, weakly; desiring another connection to the surface world.
As to how such mundane dungeons are built, that is another post.