And sometimes you just fall on your own as well. Out of the few times I’ve met the ground while biking, I’d estimate roughly half to be because I lost balance after hitting a pothole I didn’t see or ice/snow/rain making my bicycle slide. I was glad to have a helmet in these cases.

I'm not making an argument against helmets, partly because I don't know if that effect is large enough to overcome the additional safety of the helmet. I'm just trying to illustrate that sometimes there are non-obvious offsetting effects.