science

Because that's what tires are made out of, and they wear away as you drive (that's why you need to replace tires every so often as the treads wear away). That material doesn't just disappear, it gets ground down and dispersed into the environment as they're used.

It's one reason why electric cars aren't a great solution for anyone but car manufacturers—the tires. They'll cut down on fossil fuel use and fuel emissions, yes, but that's not the only way cars pollute.

EDIT: Also, thank you for asking! Always happy to explain :)