It's the most common sports related head injury by a wide margin and helmets are quite effective at injury reduction.[1] As a public health policy it makes a lot of sense. Anecdotally I've flown over my handlebars and hit my helmet without serious injury, and I'm sure I would have been in much worse shape otherwise.
On cars the law requires seat belts and airbags and a variety of other legally enforced safety measures. If you have a study suggesting that helmets would significantly help I'd be curious to see it.