The rageguy one would say either patents or "whoa the colors really pop I want that shut up here's my $$$" uncancellable LOOKATMEIAMTHESHINY mall mode, but via Occam'r razor I think mostly because they (manufacturers) simply don't care (about consumers, or about making a good product at all)
TVs/monitors (or laptops even, and more phones that you'd believe) with just a simple auto-brightness are stupendously rare even though Apple does it since forever and a half ago.
Android does have a night mode which changes the white balance of the screen at sunset and sunrise, but this is just a binary thing and doesn't respond to actual ambient light.
It's an abbreviation, and you're one of today's lucky 10000 - https://xkcd.com/1053/ for an explanation of the 10000 phrase.
No idea what "AUIU" is, but yes, generally displays should do automatic white balance like iPhones do. I don't know why most Android phones don't seem to do it (pretty sure mine doesn't), and generally TVs/monitors also don't do it. (The required color temperature sensor can't be that expensive?)