According to the National Institutes of Health, approximately 50% of all head injuries are due to motor vehicle accidents. Of those around 1% involve motorcycles. (looking for the cite, going from memory) However, the number of motorcycles on the road is about the same percentage, indicating that you're about as likely to get a head injury in an automobile than on a motorcycle. On the other hand you're somewhat more likely to suffer injury in any accident on a motorcycle due to your exposure.
Why don't we need helmets for automobile drivers? No. But more than half of all vehicle accidents involve alcohol. I think it would be wise to strengthen laws punishing driving under the influence, and requiring helmets for motorcyclists.