Many used to (pre windows ce), but writing the whole stack was more expensive than license+support costs.

Many still do, but they aren’t full fledged kiosks. By the time you get to full HD screens, the cost of the chips needed to refresh the screen in a reliable way outpace the cost of going standard consumer electronics. Cost for parts/replacement is also lower that way. This dovetails into needing an OS that supports those chips, which suddenly we are into a full OS.