It doesn't make cycling safer for the individual, it makes society safer and people healthier. Individually riding with a helmet is safer than riding while not wearing a helmet, but if everyone rides a bike without a helmet, fewer people die and people live longer because there are far fewer cars on the road.

Fewer people are driving cars which are much more lethal to non-drivers, so fewer people get killed. People are more active and live longer because of increased physical activity.

Making bikes more appealing/ easier to access by not requiring helmets makes people collectively safer and healthier.