Because churches aren't changing velocity all the time, causing your body to move around. Even with safer automatic cars, we'll still need seat belts. Have you ever tried strapping an adult seat belt onto a 2 year old?
Why are cars changing velocity all the time? Not enough input and output on the conditions outside the car.
Self driving cars don't have to drive like humans, they have to drive like 90% of the other cars. Remove the humans and self driving cars can adopt far more sophisticated traffic patterns than the simple stop/go of most city traffic.
