Sure, but the question remains whether mandatory helmets would prevent a significant number of car injuries and deaths (I'd say that's quite plausible). Whether cars have other safety features already is neither here nor there.
On a bike you are exposed. In a car, you are not. On a bike you travel around 30km average. In a car, up to 100km average.
When travelling at 100km with no seatbelt (legal in many areas), in a collision, you become exposed rather suddenly.