[mikestew]

Your keyword there is "dense". Rotating weight on a vehicle is basically: bad. But nothing is lighter than air, amirite?

John Dunlop didn't invent the pneumatic tire, but he put it into production. Dunlop's son was getting headaches riding his tricycle, so John D. softened the ride (as pointed out by sibling comment).

And so using air gives us three things that apparently haven't been found with any other material in combination: minimal weight, cushioning, and it can support the weight of the vehicle. If it needs replacing, one need only find an air compressor.