Answer:
Airbags were never intended to replace seat belts, they are supplemental restraints. Airbag design is made with the assumption that you're wearing a seat belt, so it will deploy to protect you while you're wearing it. To distribute the energy created by the impact around your body to protect it from severe impact.
Over 163 million (72.8 %) of the more than 224 million cars and light trucks on U.S. roads have driver airbags. Over 144 million (64.2 %) of these also have passenger airbags. Another 1 million new vehicles with airbags are being sold each month.
Airbags are proven to reduce the risk of death and injury from an auto accident. ... Airbags and seatbelts are safety constraints to help stop the passenger(s) without harming the passenger(s). Seatbelts, when worn, help protect the people in the car from side swipes, fender benders, and front-on collisions.
"Note also that the effectiveness of a belt-bag system is somewhat less than the sum of the effectiveness ratings of its two components. …" The study says belts are 48 percent effective in preventing fatalities, while airbags are 14 percent effective. But added together, they're just 53.72 percent effective.