Although seatbelts and airbags are safety measures used to save people's lives in the event of an accident, they can still cause some pretty damaging injuries.
Seatbelts are specifically made to confine the chest and abdomen during an accident. When a vehicle is hit, the seatbelt restricts the body from flying forward and out the window. While this may save your life, it can still leave people with very serious abdomen and chest injuries.
Airbags are meant to deploy when the vehicle is hit with a significant amount of force. They can deploy with enough velocity to literally strip the skin off of the driver's hands, as well as cause blunt force trauma to the head.
While these safety precautions are absolutely necessary, you may still need significant medical attention after being struck by a commercial vehicle.