Not all tire wear is when skidding out. A car tire's contact patch is several inches wide (especially on trucks/SUVs where extra-wide tires are often used to give a more premium look), so any time that wheel is turning a corner, there's a portion of it at the outside and inside that's rotating at a different speed than the pavement beneath it is moving.

There's also the regular deformation of wheel just in the course of regular rotation, which is where the majority of highway wear dust comes from.