IIRC vignetting is mostly lens related and it happens as the lens is not able to give even amount of light to the sensor's edges. (same ofcourse applies to conventional camera's film) If you look at very old photographs they often have strong vignetting.

In camera (or phone in this case) processing is not very good anyways as it has only small fraction of time and resources compared to doing the same on a powerful desktop with good image manipulation software. So I guess we are better off removing vignetting from raw images.