Why Is a car tire filled with gas?

Why Is a car tire filled with gas?
In the 1930's and earlier, car tires were solid rubber. But the problem was, every little bump was felt by passengers and was extremely uncomfortable, and could cause back pain. And, any delicate cargo was almost always shattered.
So, they needed a way to absorb shock. The most advanced way was by hollowing the tires and filling them with air.

Nowadays, we have more advanced methods for dealing with shock, but a lot of the time, you need to empty the air and make your tire flat to build traction, and with something shock-absorbant (such as Sorbophane) you can't do that.

And, filling tires with air is simply, cheaper.