I haven't seen anything that indicates helmets would be of much value for a driver who's properly belted in, particularly in a car with air bags. At low speeds you're already adequately protected by the car and its "crush space," and high speed crashes are rare enough and the impact severe enough that a bike helmet, while it certainly would save some lives, would be worth the hassle.
Plus people just won't DO it. They won't even fasten seat belts or learn to drive, beyond just aiming the car between the lines most of the time. If they didn't take innocent victims with them so often, I could argue that we'd be interfering with natural selection.