Nostalgia is a heck of a thing.
iPhones aren't exciting anymore because we've had 14 or so years of them now, and whereas the first five to ten years of CPU/RAM/Camera upgrades were noticeable, they aren't now. Apple users don't want exciting, well they say they do but they certainly wouldn't tolerate the teething pains which come with true tech excitement and innovation. People say they want under screen cameras to get rid of the notch/island, but they wouldn't tolerate the glimmer you see when the camera is hidden or the reduced image quality. People say they want exciting form factors but look at the reactions to the teething pains that come with folding devices or something like the curved Samsung displays. And if Apple puts a Periscope zoom in the 15 Pro, Apple users will complain about the reduced battery life because all current zoom modules are at least twice the size of the 14's main camera module which means a smaller battery will be used.
So despite all the claims to the contrary, Apple users actually want the flat rectangle that they are used to. And this is why iPhone upgrades are never as exciting as upgrades form other manufacturers.