The only reason I can think of is that the image pipeline is optimized for the new chip. The new A17 Pro can do double the ML operations:
And I’d expect the 15 to have a differently tuned image pipeline to account for the smaller sensor.
So technically the combination A16 + 14/15 pro sensor is not developed. And is marketing wise perhaps not smart to implement.
Wise they would do it. I guess these days the camera system is a 50/50 split between hardware and software. And by buying the new iPhone you are buying the latest ML optimized models with it.