The environment is full of extremely abrasive particles. It probably isn't your fingernail itself that is scratching the paint, but the microscopic grit betwee

The environment is full of extremely abrasive particles. It probably isn't your fingernail itself that is scratching the paint, but the microscopic grit between your nail and the paint which acts like sandpaper. You drag those particles across the paint and they dig in like tiny cutting tools. This is the same reason that keeping eyeglasses and other optics scratch-free takes careful cleaning technique. One careless wipe when there is the wrong kind of dust on the lens or on the cloth, and you gouge the surface.