Because Apple is largely about providing the best results for most people in as automatic a way as possible.
If you asked most people, they would tell you they love the photos the iPhone 13 family of devices take.
You want something different, and by and large, Apple leaves "different" up to third-party providers.
The fact that the photo data is available to third-party camera apps tells you they're leaving the controls up to those who are interested in having them to implement.
A great example: for years, Apple did not make many exposure controls available in the Camera app; now they do.
An example of functionality they do not provide: despite being asked for years, there is no way to shut off OIS, which is a problem for anyone using an iPhone with a gimbal like a DJI Osmo.