Because the majority of helmet wearers recognise that in the type of incident that most cyclists are likely to encounter (minimal impact/dermabrasions) even a cheap, disposable skid lid hitting something hard before their head/face is, well, a no brainer...
...someone, somewhere above suggested that any given bunch of stat's are skewed due to the non-reporting of minor incidents and they hit the nail fairly and squarely on its non-helmeted head; I have taken a tumble on numerous occasions and scraped, gouged or dented a riding/cycle/crash/para/cycle helmet in the process but not once required medical attention (sectioning may have been more appropriate in hindsight!
) and certainly never reported them anywhere, until now...
...which is the point I think a lot of people are making.
Without a helmet I would have left a considerable amount of skin/hair/blood scattered half way around the world that I am glad to be still attached to!
And whilst I am more than capable of Rambo'esque
DIY surgery I'd much rather not have to prove it these days
I am trying to be very specific in stating that current helmets appear to have a problem absorbing the energies involved in certain types of low energy impacts that cause head injuries and concussion. I have no doubt that current helmets do provide protection from lacerations, grazing, bruising etc. Or in fact from linear impacts of between 7mph and 14mph against flat surfaces. I think that the misunderstanding here is because of the number of possible interpretations of the term "head injury". As far as I can remember the term "head injury" in the graphs I have posted refers to hospital admissions and not just visits to A&E. And as such they would not, as you and others have said, Indicate how effective helmets are in preventing minor injuries.