I imagine it has to do with the natural way humans evolved to survive, with a wider peripheral vision over a taller one (eyes side-by-side instead of on top of each other), so we could see more threats on the horizon (birds used to be natural predators to humans—babies mostly—but over time they’ve become less of a concern to us).
It’s also easier to shift your head side-to-side instead of up & down.
Do you have one of those friends that films everything on their iPhones in portrait, instead of landscape mode? Isn’t that annoying? It’s because you can’t see what’s “around you”.
Also, books are “widescreen”, too, in contrast with the older scroll format.
With responsive design, and other evolving web technologies, we should be able to craft better horizontal experiences on the screen, so pretty soon the idea of a tall screen is going to seem more unnecessary, and you won’t have to be That Guy at the office who has his computer monitor on its side.