16:9 is a compromise aspect ratio between older movies that were 4:3(going all the way back to black and white film) and 'modern' widescreen formats that sit around 21:9. Thats why you see the black bars at the top at bottom on 16:9 screens displaying 21:9 content.

I say 'modern' widescreen formats like 21:9 because the aspect ratio wars kicked off in the 1950's with Cinerama, which was three 35mm cameras taped together and required the same 3 projectors in the movie theater to work. Ridiculous complexity and cost. The war ended with some clever anamorphic lenses letting filmmakers use just the one 35mm camera(and therefore single cost of film) and using the reverse lens on the projector in the cinema.

This was all in effort to destroy peoples experience in watching movies at home on TVs(so they would buy movie tickets instead). It didn't work; people still did that, they watched 21:9 content on 4:3, which then had to be 'panned and scanned'. If you ask any director or DP about panning and scanning, you'll witness an entire horror movie play on their face. Or they'll just start swearing before you've finished saying 'scanning'.

The effect of P&S is taking a 21:9 movie like ben-hur, then having a teenager record that movie on their phone in vertical mode(9:16), trying to keep all the principal action in frame. A man named Ken Powers came along and established a compromise between 21:9 and 4:3 = 16:9. This is a now the dominate aspect outside the movie theaters, unless you buy specific ultra wide screen monitors or home projectors.

"For both eyes the combined visual field is 130–135° vertically[33][34] and 200–220° horizontally."

https://en.m.wikipedia.org/wiki/Peripheral_vision

That means that the ideal screen ratio if you want to fill up someone's field of view while they are looking in the middle at a good distance is 0.61-0.65. 9/16 is 0.56 which is that far of. 1:2 would be 0.5 which is worse. 1:1 would be 1 which is very far from the ideal.

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.

Just to add to the others, the 2:1 aspect ratio is actually becoming more popular with cinematographers, presumably because the image is a bit wider and more "cinematic" than the standard 16:9 format.

For example Stranger Things and House of the Dragon are shot in 2:1, and reportedly it's the fastest-growing format in cinema.

Edit: Removed Star Trek since only Discovery S1 was shot in 2:1, after that they used 2.39:1

Is it just luck that 16:9 is 4:3 cubed. I'd always assumed there was a reason though never really investigated the thought