Caves are a natural drawcard for adventuring types keen on exploration, adventure, and the promise of piles of gold.

In reality caves are dank, dark, dangerous places, which can be either numbingly cold, stiflingly hot, or filled with noxious gases. There's a very good reason cavers wear hard helmets. Yet explorers in games seem highly unlikely to end an underground exploration unconscious and bleeding from the skull after cracking their head on the ceiling.

Presumably most caves found in game scenarios have been modified and made comfortable for living in by whatever hordes of goblinoids happen to inhabit it.

But then if you want to be cruel, remember that goblins (at least in Dungeons & Dragons) are only about a metre tall. They'd be perfectly comfortable with a ceiling 1.5 metres high.

Transcript

Luke: I'm going to check out the tree.
Yoda: Ahh ha ha ha! Your laser sword, better if you leave it.
Luke: Oh, really? In that case I'll take the blaster too.
GM: You see a dark cave under the roots.
Luke: I climb in.
Yoda: {to himself} By the pricking of my toes, something wicked that way goes.
GM: A sleen monitors the entrance.
Luke: What's a sleen?
Han: It's the organ behind the splanch.
GM: It's a... monitor—
R2-D2: Oooh! Wait, I can patch into the auxiliary data port and—
GM: —lizard.
R2-D2: Oh.