I think it's an ergonomic issue. Phones (even the Pro Max size) can be held with one hand or two hands without resting your palm or pinching the edge to hold it. You could but it could cause some erratic behavior.

A tablet though doesn't hold well when just pressing on the sides. So having some place to grab and rest your palm is more necessary here. They probably could go thinner with borders but it's a balancing act of usability and aesthetics. Also have things like the camera to account for and on tablets you don't have to make a punch-hole or teardrop. The iPad Pro's also package in FaceID cameras so it could be a product consistency choice too.