In bike-friendly places like Amsterdam and Copenhagen, it's super rare to see a helmet. A bike helmet offers reasonable protection in a solo fall, but it has nearly cosmetic effect if you're hit by a car.

But one thing you don't see in those places is huge, flat-front trucks with poor visibility and worse pedestrian impact safety. Until the US mandates pedestrian safety measures, we won't see a reversal in this trend.

Edit: I mostly bike places here in California, and I do wear a helmet 99% of the time, but I don't judge those who choose not to, because the biggest factor in bike safety is having other bikes on the road.