Rubber deposits and dirt from the blades
Waxes and silicones from car cleaning products, e.g., from automatic car washes.
Atmospheric pollution, insects, grit, road spray including salts and exhaust particles.
Scratches on screens form minute “reservoirs” for these contaminants.
Wipers spread them, especially when used dry, i.e., without detergent from the washer
Excess Rain-X on screen