Apart from the fact that a lot of systems still have mediocre high-dpi support at best, there's also battery life, cost and the fact that 1080p on a 15" is more than adequate for most people.
As far as I can tell, the only desktop OS with good high-DPI support is macOS.
The high DPI suppport on gnome equates to 'SCALE ALL THE THINGS 2x DERPA DERPA DERP'
And the scaling is integer-based. So, I can choose 1x, 2x, or any other whole number. Useless as hell for me, who wishes it was probably 1.25x.
It sucks bad enough that I actually WISH I was on a 1080p screen, because with the 2x scaling i effectively have a 1324x768 pos screen.