The way my brain rationalizes it (inverted y, normal x) is that the closest analog to my hand on a mouse is my hand on top of my character’s head.

To make that head look up I pull my hand back, which is the same exact motion as pulling the mouse back. So it feels natural.

To make the head look left, I would rotate my hand counterclockwise. Rotating a mouse doesn’t do anything, so I have to translate that to lateral motion, and left to look left feels more natural.

Of course the real explanation is that the first mouselook games I played defaulted to inverted y and normal x, so that’s what I got used to. And even before mouselook became a thing, I was playing flight sims, which default to inverted y. Still, it’s fun to try to rationalize something that ultimately boils down muscle memory.