Because long lines are bad for splits and/or vertical monitors. You don’t know the viewport the code will be read in, so keeping to a conservative width is helpful. If things wrap or require scrolling, that’s a lot worse for legibility, whereas having lines limited to 80 has minimal impact on legibility.

I personally format all comments to 80 (using vim’s gw command) and never touch LLMs.

The difference is that there were services that could leverage the Internet and be profitable. Every AI company is wildly unprofitable and burning unfathomable amounts of money. There’s no path for them to become profitable, unlike some companies in the dotcom era.