The iPhone has a relatively wide fixed aperture. For example, on iPhone 7 Plus, it's f/1.8 for wide-angle and f/2.8 for telephoto. Since the aperture is fixed, according to exposure triangle proper exposure must be obtained only by adjusting shutter speed and sensor sensitivity. Fastest shutter speed 1/8000s is similar to what you'd see in a larger camera, so the ISO has to go lower for capturing those sunny moments.
A prosumer camera can rely on choosing a smaller aperture. Also, buyers of prosumer cameras are more interested in low-light performance, so is considered more important by any camera buyer - the bigger sensor found in prosumer cameras is optimized towards higher sensitivity range as it allows physical room for that (bigger sensels).