Level drain and dragons

Why do mere undead get the “scary to the players” out-of-game-world mechanic of level drain* whilst the freaking dragons have to make due with mundane attacks?

Let’s start with the fact that flying and breath weapons are pretty impressive. Certainly PCs get to the point where those things aren’t so impressive anymore, but at that point the PCs aren’t really mundane anymore.

Secondly, there’s a good argument that dragons weren’t meant to be ultimate powers in D&D.

Thirdly, dragons aren’t there to represent the supernatural. Although we know that the cube-square law means that D&D dragons would have to be supernatural, in the milieu of the game, they aren’t. The out-of-game-world mechanic of level drain is justified to represent the sort of irrational horror of coming face-to-face with powerful supernatural beings represented by level-draining undead.

Or, at least, that’s my thinking aloud for today.

*I actually play it as XP drain, but I’m not going to call it “energy drain” because it isn’t about any diegetic rationalization. It is about simulating a horror that is best simulated by a extradiegetic mechanic.