I don't fully know but it is probably something to do with the fact tires are designed to be more malleable and flexible than roads. The tire is also flexing an

I don't fully know but it is probably something to do with the fact tires are designed to be more malleable and flexible than roads. The tire is also flexing and pushing on an air cushion while the road itself is being pressed against and having to flex with the ground. Also, almost all the particulate emissions are due to the abrasive nature of the road-tire interface tearing apart the tire. Tires are a cheaper and simpler wear items than roads, something is going to give, so we've decided we'll replace our tires more often than tearing up our roads. Just like if you ever got road rash, the road is going to tear you up far more than you're going to tear the road up. So, while the road forms cracks and what not from its repeated stresses it's not coming apart like dust nearly as much. Don't get me wrong, some small, tiny amount of it does but not nearly as much as the tire.