The best way to protect a head from injury is not to hit it or have it hit in the first instance. A helmet cannot prevent a fall or a collision, so a cyclist’s first line of defence is to reduce a likelihood of a fall or collision: (A) Air in tyres - correctly inflated (B) Brakes working correctly (C) Chain running freely (D) Danglers such as laces or trouser legs tucked away. Communicating effectively with other road users such as by arm signals, eye contact and road positioning all reduce the likelihood of a collision.
Only when you have done all you can to reduce the likelihood of a fall or a collision should you consider secondary measures to reduce the consequences of a fall or collision.
Unfortunately there is evidence that wearing a helmet encourages risky behaviour by cyclists, especially young cyclists, for example by trying out tricks: pulling wheelies or riding without holding the handlebars.
It also seems that drivers take more risks around helmeted cyclists, by passing closer or faster. This is partly evidenced by your post which in essence says, “unhelmeted cyclists! How vulnerable!” (I must take care in passing them.)
A recent Dutch study found that 13% of Dutch cyclists admitted to hospital for head injuries were wearing helmets. Helmet use in the Netherlands is 1%.
If pushing the limits, by for example, racing, off-roading, BMXing wearing a helmet makes good sense. For utility cycling or leisure cycling it is purely a matter of personal choice.