Why do vertical lines as eyes seem more natural than horizontal lines, even though in reality eyes are more horizontal than vertical?
Probably because of cats and shit.
But also, the almond shape of eyes make it so the cornea touches the top and sometimes bottom lid and is interrupted either side by the sclera. So it's better represented by a vertical line touching the top and bottom of each lid, than a horizontal line suggesting it stretches to either end or that the eye is closed.
I guess because the iris and the pupil become a vertical ellipse when the head is not viewed from the front.
Actually it's really simple. Our minds work in symbols, always have always will, when we see a vertical line it implies that they are wide open, attentive and appealing, much like a Disney cartoon's appeal, the character looks like it's aware of what's going on, horizontal implies either hooded, half closed, sleepy eyes, which can work fine as well, but the viewer interprets less energy and thus it loses some appeal that we look for in a simplified style that would want lines for eyes.
There's also tangents going on but the fact that they're horizontal or verticle doesn't distort the face, it's what the artist does WITH the face that would distort it, not the other way around.