Because it doesn't deliver the same result! Otherwise the iPhone wouldn't need 3 different lenses, it could just use a single one and internally convert everything from 48MP to the selected mode. The reason it has 3 lenses is that they're basically 3 different fixed focal length lenses: The 48MP mode is equivalent to 24mm, whereas the 5x tele is equivalent to 120mm.
In other words, depending on the mode the light travels different distances, hence the different focal lenghts. For the 15 Pro they have this folded tetraprism in the tiny space between sensor and housing to increase the focal length up to 120mm.
If you take a 48MP photo as you suggest you can't use this mode, so for example you'd end up with a portrait shot at 24mm which will of course look horrible no matter how much you zoom later, it will just blow up the image.
Well that’s exactly what the extra lenses are doing with the 1x camera. The 1x camera will always remain at 24mm equivalent focal length. Same with the ultra wide, it’s 13mm.
The 120mm camera works on its own, not cropped in any way.
They’re not actual extra lenses, only “borrowed” areas from the 48mp sensor. The 1x camera is using 1x=24mm, 1.2X=28mm, 1.5X=35mm cropped or zoomed in images. Here are the screenshots from the Apple event. You can clearly see the zoom increments per focal length selected. These are extra focal lengths from the 1x generated by software.
If you recall, the iPhone 14 pro / max has the 2x camera, which is a crop from the 48mp sensor to give you the 2x or 12mp file. I suppose that will be equivalent to a 42mm lens.
The iPhone 15 pro max is a fantastic looking phone, I’m not shooting it down at all, I’m only pointing out what they are doing with the 1x camera. i don’t need a 5x camera, the 3x is plenty. I wish the sensor was 24~36mp, but it’s still 12mp. I’ll probably buy the new iPhone next year around march or April (possibly). But right now I don’t think the 15 pro max has enough upgrades for me to consider one at the moment.
So there you have it. Images are cropped at 1.5x and lower.
And I just duplicated the exact same thing by holding the 1X camera button down and zooming into 1.2 X and 1.5 X and getting the exact same results without any distortion. Try it yourself and see. The 1.5 X gives me 20 megapixels of image resolution.