Proximity sensors for cars

I saw someone backing her car into a parking spot yesterday. She managed to get the fender of her car onto the footpath. Luckily for her, the car cleared the footpath, so there was no damage done. The thing is that she didn't even know; a driver from another car had to tell her. I've had this problem sometimes when I've reversed a car too far. Figuring out your car's length (depth?) is not so easy and it does take some driving experience. So, it's surprising that cars don't have proximity sensors at the back to tell you when you're too close to something. I think that some "luxury" cars do, but isn't a proximity sensor something that could benefit everyone? Wouldn't it make reversing so much easier for drivers, inexperienced or otherwise? Why is there such a surprising lack of innovation or even attention to things that would help make a user's job easier?