Because Folks aren't people, Mettaton. It's an actual term.

[He drags a hand down the front of his face, exasperated and feeling a distinct sense of shame an embarrassment that he can't make go away. This whole thing is a disaster. How could he sleep walk? How could Mettaton have followed him to hear him say...goodness, whatever nonsense he said?]

They're monstrous souls of the dead. From the Netherworld. And I don't, uh, destroy them, not really. It's...complicated. I'm not killing them, I just...

[God, this sounds so weird.]

I...well, I absorb them.