Bike rides off the side of the road, fall over, hit head on a rock.

Run head on into a pole.

Certainly, the style of bike helps and those are safer bikes... they don't prevent head trauma though. A helmet does.

This is also similar with skiing. I learned to ski when I was a kid and never wore a helmet. People wear helmets now. Even on bunny slopes. Kids wear them too. You're on snow, it is soft. Helmets are still a good idea.

It is the accidents you're not expecting that tend to hurt you the most.