I saw a video where first research papers were proposing widescreen (16:9) for wide adoption, with also 5.1 sound. This goes back into late 80's or early 90's. There was a lot of science and testing done to end up with 16:9 being generally the best screen aspect. It uses more peripheral vision than 4:3 while not going overboard. While I can understand the nostalgia and great 4:3 screens, it's standardization was born from necessary compromise with the cathode ray technology and manufacturing limitations with first tubes. Widescreen CRT's come after decades of advancements in the tech after all, with their own issues. Ultrawide isn't good for TV's. They have to be curved for best experience and there's a sweet spot with UW monitors. A curved TV (ultrawide or not) would suck for a group or a family. This leaves UW as a niche enthusiast monitor choice for PC users where monitor placement and sweet spot are easy to manage, lone use is the norm and there is also some productivity benefit with 2-3 normal monitors in one. I'd imagine car simulators and FPS games rock with UW, not ever owning one myself. But what benefit do 3rd person action games, retro games, fighting games, side scrollers and such get with UW? No video content benefits either aside forcing 1.8:1 or 2.4:1 movies to fill your UW screen. Phones have wilder aspect ratios now, but the vertical & horizontal dual usage of phones and other considerations make the logic more malleable. Portable devices in general escape logic devised for large TV's.
Heck I want more weird ways to use vertical screens. Shmups for 16:9 screens oriented vertically. Funky stuff they did and still do with multiple screen arcade games like Dariusburst, Xmen deluxe 6p cabinet, ninja warriors etc. Can't be THAT wild with these standards but I wish more phone games were vertical and playful with it. TATE me out bro.
TL;DR:
4:3 was born mostly from technological necessity, a compromise, likely.
16:9 had a lot of thought put into it, a long time ago, tested and calculated out as best big screen aspect ratio for human eyes. We have not replaced it in several decades for a reason.
Ultrawide has limited universality and other tradeoffs, thus limited to a niche PC monitor choice for enthusiasts.
"Lol" said the phones. "Lmao"