Because making it work is burdensome and writing programs that interact with it even more so. I'm guessing you've never tried to write a terminal rendering pro

Because making it work is burdensome and writing programs that interact with it even more so. I'm guessing you've never tried to write a terminal rendering program. The hoops you hav to jump through to get vi to switch into a blank screen and then drop back and re-render your previous terminal. Behaviour differences on some terminals when you run man and the previous output is simply cleared or the man page is printed and scrolled. There are piles of hacks.