Why aren't tires made of solid rubber instead of filled with air?

Though it may seem counterintuitive, the fact is that tires filled with air provide a much smoother ride than those made of solid rubber.

The benefits of air-filled tires were first discovered during the early days of the automobile, when drivers quickly became tired of the bumpy ride provided by solid tires. In order to improve the comfort of their ride, they began experimenting with different ways of filling their tires with air. Though the exact details are a bit fuzzy, it is clear that air-filled tires soon became the standard for the automotive industry. Today, though solid tires are still used in some industrial applications, they have largely been replaced by air-filled tires in the world of passenger vehicles.