Literally no kids of a mid-size town in Bulgaria had no head injury riding a bike w/o helmet since I was kid. Not sure where you got the two or a dozen. In fact, a dozen is a fairly good sample size in terms of medical studies, sometimes done with conclusions done with smaller cohorts. In fact, the first COVID-19 booster by Pfizer was only tested on mice, not on a single person even, yet it was administered to billions of people.

Helmets and bike gear are in industry and American industries have powerful lobbies.