It's a matter of risk assessment. The chance of a healthy adult cycling in the Netherlands getting a head injury that would have been prevent by a helmet are so incredibly low, that making it a requirement would have a detrimental effect on the collective health of Dutch society due to slightly lower uptake in cycling.
For elderly and young children a helmet may make sense, due to the significantly higher risk of such injuries (due to reduced physical responses and inexperience). And for racing bicycles wearing a helmet is the norm.
A helmet can significantly reduce the impact of a fall, but the risk of that fall is so low that the measure becomes statistically meaningless. It is akin to always wearing a hard hat when you leave your home, because it would save you from serious brain injury if a brick fell on your head. Of course, bricks don't generally fall out of the sky, but you can never be too sure…
This doesn't mean that all safety measures are nonsense — having working lights and functioning brakes do reduce the chances of accidents. It is also tempting to compare the helmet to seatbelts in a car, but there the chance of needing them is simply much higher.
One well-researched source on the negative effects of bicycle helmets if made mandatory in the Netherlands (in Dutch): https://www.fietsersbond.nl/nieuws/fietshelmen-en-veiligheid...