Well they still have to attach to the edge of the display. Probably there is a lot of stuff going on at the top of a phone (cameras etc) already. I expect it would also be harder to attach the display connector along a notch cutout.
Xiaomi got rid of the forehead entirely by moving everything to the (relatively small) chin on Mi Mix 2. While that is not great for the front facing camera usage since many (most?) apps don’t support 180 degree rotation, I don’t see why the design can’t be flipped. Just build a phone with a reasonable sized forehead. Although I don’t particularly mind the design choice for Galaxy Note 9 or Nokia 7 Plus — a narrow forehead and chin.