Do you wear one while driving your car? Exactly the same argument applies. Or when walking around? Ditto.

If you engage in any activity long enough you will die while doing it. That doesn't mean it requires protective clothing. If we try to eliminate all the tiny risks we run, our time will be less enjoyable and our health almost certainly worse than had we simply got on with being active, risk-taking people. One person in a thousand will avoid a severe head injury while the rest of us live shorter, less healthy, more miserable lives.

And that is without even considering the fact that evidence suggests that helmets don't make much difference to whether cyclists get hurt or not...