Even at steady state cruising the the tires are flexing and stretching with each revolution. Tires are round but a driveway is flat. If you look at the contact patch of the tire it flattens a bit to confirm to the driveway's shape. Now visualize with each revolution that flatten part of the tire is cycling around the wheel.
At a microscopic level there's a lot edges in the road surface that we don't fully appreciate which also contributes to tire wear. http://insideracingtechnology.com/tirebkexerpt1.htm