Movies.

Movie makers like having a nice wide backdrop, it gives them a lot of room and, since we mostly operate on a (nearly) horizontal plane, there's a lot more interesting things happening to the left and right than up and down.

There a number of particular wide formats, it doesn't *have* to be 16:9, but that's why we generally like wide formats.

We got stuck with 4:3 for years on computer monitors & TVs because it's *very* difficult to make a widescreen CRT tube. They want to be 1:1 for technical reasons and 4:3 was about as close as they could get to a widescreen format that would look movie-ish.

Fast forward about 80 years and the technical constraints went away...modern LCD/LED screens don't care what shape they are.

16:9 got settled on as the standard format for DVD/HD TV as a nice compromise where virtually all widescreen movie (and now TV) formats look pretty good with minimal black bars for the weird movie formats, so now essentially all displays are 16:9 for maximum compatibility.