Tires aren't made exclusively of natural rubber, they contain synthetic polymers among other things.
Additionally, by vulcanizing the rubber, three-dimensional chemical bonds resembling those found in synthetic plastics are created to harden the material.
So the end product is not really "natural" or "rubber" anymore.
The heavier the vehicle the more wear on the tires, and that works the other way around too. Bicycles are so lightweight that they don't shed rubber much faster tennis shoes.
