Opportunity cost of safety improvements to vehicles. Airliners cost like $100 million each, so they can afford to perfect safety. You can’t get airliner-level safety in a vehicle that costs less than $50,000.
Maybe not airliner level of safety, but based on the top causes of car accidents, they could probably be reduced by an order of magnitude with relatively inexpensive improvements:
1. better driver training
2. breathalyzer ignition interlocks
3. speed limiters (even more reduced speed at night/rain)
4. traffic light detection/automatic braking
5. enforced following distance
(I'd add "sleep/impaired driver warnings", automatic emergency braking (including pedestrian detection) even though it doesn't quite relate to the top causes of accidents))
Here's the top ten: