Because looking is always better. By the time the sensors pick something up, sound the alarm, and you react, it's usually too late. Plus they are unreliable for the most important obstacles: children. Many kids have been backed over by cars with sensors. The camera lets you actually see what's back there, and react quicker.
For the front, sensors would be a nice addition, since you're already looking, and just need to know when to stop before contact. CX9s in Australia have front parking sensors.