Instead of one small part of the phone being slightly thicker, a part where you hand doesn’t even touch and it genuinely doesn’t matter, you’d rather the entire phone be thicker, making it take up more space in you hand and pocket. Then you’d have to do something with all this extra space you’ve wasted, so the phone will have to get heavier and use more materials to make.

And in return for worse ergonomics, cost, space efficiency, and overall aesthetics, all we get is a… smooth back? Which amounts to an ARGUABLY improved different aesthetic that affects less of the overall design than the thickness does? That sounds like the worst trade deal in the history of trade deals. That’s like putting square tires on a Jeep because it arguably matches the overall shape of the vehicle lol. Especially since most people have cases on their phones which could easily make up for the difference in thickness anyway.

What would you fill all that extra space with?

Empty air? How is having empty air inside the phone better than having empty air outside the phone?

A larger battery? So you can charge every second day instead of every day? I don't think that'd be an improvement, it would almost certainly result in forgetting to charge your phone and you'd be more likely to have it go flat on you. Also batteries are extremely heavy.

A silicone protective coating? Also known as a phone case. This is really the only one that makes sense, and you can already do it.