The human skull, while not infallible, is designed to protect the brain from injury in most typical falls etc that an unassisted human body can manage.
Cycling, even at very low speed, instantly increases the energy available and takes the wonderful engineering of the skull beyond that which it has evolved to protect the brain.
Therefore (pretend for one moment that cycle helmets are highly effective) walking etc are activities for which body has adapted very well, and for the vast majority of people the skull protects the brain throughout their lives very effectively, making helmet use unnecessary for normal activity. Cycling, however, is an unnatural activity and a fall from a bike causing the head to hit the floor or another vehicle is much more likely to exceed the performance parameters of the skull, thus making a helmet of much more utility.
Until a cycle helmet arrives that genuinely works, I am anti compulsion, and the 'well pedestrians should wear helmets then' retort is completely cringeworthy.