Why is it that most text in the world is written horizontally?
It may be the case that horizontal movement is a tiny bit less costly than vertical movement; only two muscles are involved in horizontal eye movement, but vertical movement involves four. We do have to consider that, first of all, even though I wear glasses I’m not an eye expert, and second, that eye movements are so tiny that any extra exertion from vertical movement as compared to horizontal movement is likely negligible. Still, even the tiniest things can add up over time, as we learned from Office Space, and writing systems have developed over millennia.
Another possibility is our environment. (Of course, this is related to the previous one, given that biological organisms evolve in response to their environments.) Imagine living in the pre-skyscraper days, when we still had, all around us, what some people refer to as nature. Turn around, and what do you see? Sure there are trees and hills and houses poking up here and there, but there is one thing that unites the entire scene: the horizon. The great divide between earth and sky. What we focus on when we gaze into the distance. (You do gaze into the distance, don’t you?) I might even say it’s central to our existence. We like to stand on even ground. Many people are afraid of falling.
From this perspective, it only seems natural that our writing should go horizontally. (Horizon is built right into the name.) Perhaps the real question is: Why do some weirdos write vertically?