This only began to become a problem with the introduction of e-bikes. Old people especially are not wearing them on e-bikes. On regular bikes where you are going 15km/h you should not have to wear a helmet. The infrastructure in the Netherlands protects cyclists enough (in general) and a fall at that speed is not really that dangerous (in general, again).
Though an e-bike goes 25km/h. This is a lot more dangerous and people should definitely wear a helmet when driving one.
As i said, in general, going 15km/h is not fast enough to have a serious fall on your own. Only from external factors can riding a bike be dangerous, like from a car for example. in The Netherlands almost all cycle paths are isolated from cars as much as possible. So in general (again, in general) it is safe enough to drive around without a helmet.
The data backs it up. Look at the deaths per capita in the Netherlands. You can see a steep rise with the introduction of e-bikes, but before that it was one of the lowest in the world. And that is saying something when it is the most cycling dense country of the world.
who goes a max of 15 km/h on a bicycle? You'd get passed by a very fast runner. I easily go 25+ on my fully loaded bikepacking bike on typical commuting terrain.
My wife enjoys telling the story from her time living in the Hague of watching drunk girls in mini skirts all attempting to ride side-by-side to keep each other upright, and...somehow managing to do it.