There is a small percentage of people who do not use a case. I do not use a case.
Secondly, presently there is still bezel shown with the vast majority of cases. The bezels would have to be reduced about 75% more at least, before most cases covered everything but the screen.
Any cases that covered a tiny portion of screen would be of zero consequence. All people who do not use a case would enjoy the even more immersive screen, and all users with a case would not see only a tiny fraction of the screen, all of which has no touch sensitivity and holds no UI elements. That screen is strictly for immersion.