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.