Probably because the screen is 1080 (I don't even think it's that high of a resolution?).
Why would you want to do that if it's higher than the resolution of the screen?
Just dug out my 6s + to have a play with to decide whether I want to move back from Android.
It seems YouTube is capped at 1080P and I can't view anything higher than that.
Google uses the standard H264 for YouTube videos up to 1080p, but uses their own VP9 for 4K. Apple does support VP9 starting in iOS 14 but only for devices with the A12 chip or later.
So iPhone XS and newer, and iPads with A12 or newer do support YouTube 4K.
On an Android though you can crank it all the way to 4K even if you had a 480p screen.
It used to be capped on Android as well, it was only recently they decided to allow all devices to pick whatever resolution they want.
Wasn't the case for me on a couple of older devices. It was only officially rolled out last year: https://m.gsmarena.com/youtube_for_android_showing_4k_playback_option_for_some_users-news-47842.phpI had my Pixel 3 XL, 3 years ago, and I was always able to choose 4K resolution
Wasn't the case for me on a couple of older devices. It was only officially rolled out last year: https://m.gsmarena.com/youtube_for_android_showing_4k_playback_option_for_some_users-news-47842.php