If you filled the negative space on an iPhone 17 Pro/Max it would be a horrendously unwieldy phone. People who say this stuff seriously underestimate the disastrous effect on ergonomics increasing the whole phone from 8.75mm to 13.2mm would have.

Would only need to be at the top, to balance out the camera.

The only reason I use a case is that the iPhone is close to unusable on a flat surface without it.

The iPhone is already heavy as a brick, and a battery is a couple orders of magnitude heavier than a TPU shell covering the same volume.

People already do this by buying cases

Most would continue to buy cases which would only make these phones even thicker

I buy cases to make my phone feel thicker around 12mm. I have no idea what you're taking about.