Because they do not improve outcomes when a driver runs you over, the speeds involved are simply too high. When you do mountainbiking, a helmet protects you, speeds are lower, but not on the public roads.
I cycle in a city and a know several people who have been hit by cars. None of them went under the car’s wheels. Half of them believe their helmet saved their life. It’s not as though you automatically die upon contact with a car, and of course it helps to have your head protected.
Just like getting hit by a car happens almost never? Anecdotally, I have never been hit by a car, but have fallen off my bike multiple times for a variety of reasons: bike entered the lane without warning while I was going downhill, I hit a rock on the road, front tire got caught in a groove in the road, …
Falling off because of gravel on the road, grooved surface or other road hazards sounds too much like inattentive riding or speed unsuited to road conditions. A helmet would help in those cases, but better attention to the road would also be a good idea.
