Foldables need to have very thin construction as when folded, you essentially have two phones on top of each other. There's speculation online that the air was launched off the back of their internal foldable R&D.

They stacked almost ALL the actual stuff, like the camera system, SOC, etc. inside that little plateau and the rest of the slab is basically just display and battery. If I was going to make a clamshell phone, experimenting with miniaturizing and arranging the whole thing into a small corner of the footprint would be where I’d start.

If you scroll halfway down the press release page you can see an image of the internals https://www.apple.com/newsroom/2025/09/introducing-iphone-ai...

Presumably because a folding phone needs each half to be quite thin to still produce a reasonably normal phone thickness when folded.