According to the CDC, 30% of injury related deaths are TBI-related, and 14% of those TBIs are from car accidents. Why not have mandatory helmets for auto drivers too?
It's all irrationality and a belief that cars are much safer than we pretend they are.
As someone who has worn a helmet in the car from time to time, a lot of cars don't even leave enough headroom available for someone 6' tall to wear the helmet without their head hitting the ceiling.