Why don't motorcycles have seat belts?

The purpose of a seatbelt is to keep you from being ejected from your vehicle because in your vehicle you are protected by the cacoon of the vehicle frame and body. Outside the vehicle you may either be run over or come in direct contact with another object. On a motorcycle you are not in any kind of protected environment and are already exposed to the other objects in the area. Staying with the motorcycle offers no safety advantage.