You have to understand the complete working of an air bag before you can ask this question. The most important units of an air bag system are: an air bag and gas generator, crash sensors and a diagnostic monitoring unit.
In case of a collision, which is taken to be the car hitting a barrier at 14.48 km/hr, the crash sensors detect the deceleration and send an electric current through an initiator. The initiator leads to a solid propellant undergoing a rapid production of a gas, usually nitrogen, which fills up the airbag in less than 1/20th of a second. The bag remains inflated for 1/10th of a second and then gets deflated which takes around 3/10th of a second.
This is a very essential part of the operation of the airbag mechanism. As you may guess, just an inflated bag is going to cause a lot of harm if you crash into it, just like you will injure yourself if you fall from a height even on something soft. The fact that the air bag inflates and then deflates after a very short duration of time allows it to absorb the energy of the impact and protects you in case of an impact.
Air bags thus increase the deceleration time and so decrease greatly the force on the passenger.