There's a lot of fiddly details to fully cover VGA. And very few people using it still, so there you go. Kind of a shame, because some of the features are super handy for hobby OSes; scrolling the screen by moving the start address is so much nicer than scrolling by copying, but you can't do that with a frame buffer that UEFI set up for you. Text mode is handy too, although 8-bit characters is problematic; UEFI's boot services has a text mode console, but when you exit boot services, you've got to render text on your own.