Dirt does not stick to scars because they have no hair follicles or sweat glands
Scarring is the body’s natural response to skin breaking. It is curious why those healed-over patches look so smooth. The thing is that while the fibers in regular skin grow in random directions, the ones in scar tissue form facing the same way.
The new skin grows so fast that it does not have time to arrange itself in a crisscrossing pattern or to develop hair follicles or sweat glands. That is also why dirt does not stick to it as it does to the rest of your dermis.