Because cameras take snapshots in a very short amount of time. The stars are simply not bright enough to show up in the pictures because if you leave the shutter of the camera open long enough to collect enough starlight, the picture would be washed out with other, brighter incoming light. Take your camera phone outside and look up at the stars. Then take a picture and you will see no stars. They aren't bright enough. Images you see of starry skies are done over long exposure times in places where it is otherwise totally dark so the shutter can stay open longer. Many times these cameras have to be on a special tripod that moves to track the star's motion due to the rotation of the earth. If the camera is left still, the stars in the image will look like streaks, tracing out an arc through the sky.
