What made you go WTF today?

[Khendraja'aro]

Let's see, head injuries happen in car accidents too. Maybe everyone riding in a car should be required to wear a helmet. Oh, can't see to the sides? Tough shit, safety first! Can't hear other vehicles? Tough shit, safety first! Limited head mobility? Tough shit, safety first!

Inside a car, you don't brake with your head!

You wear seat belts in a car, because then you are "fixed" to this car. Which means that if the car brakes, you will decelerate at the same rate. With the crumple zone available (about 1 meter) this means that the deceleration just was reduced to a mere 30g (@~90Km/h) - while that will break bones, you also will survive that.
If you don't wear a seat belt, you will be catapulted out of your seat and usually through the windshield. Which obviously is a bit more problematic to your health.

It's the same principle with a helmet - I can calculate the g-forces you're subjected to with and without a helmet. You'll see that even a mere few centimeters will make a difference between "bashed-in brain" and "evil headache".

This is physics.