Tiny lenses and tiny image sensors means the camera can't gather that much light.
Less light means the signal-to-noise ratio can get pretty low, which leads to lower image detail.
This is why you're now seeing phones with multiple cameras on the back.
The multiple cameras can be combined to form a 'synthetic aperture', which means that the multiple smaller lenses are combined to make a single larger lens in software.
It's pretty neat, but it's hard to beat an actual camera.