Liquids act as lubricant. All abrasive dirt is wiped off. Rust in an important life limiting factor in regular spring steel. Since it will not rust, it will keep its strength. Another important life-limiting factor of a spring is the number of cycles it will withstand.
Failure generally occurs after one million cycles.
Wiper arm springs are not subject to such cycles as would suspension springs for example. Wiper arms are found in a wide variety of sizes and shapes. Over 1,000,000 pairs have been sold to satisfied customers. In hot weather, the oils in road asphalt rise to the surface and are projected onto your windshield by other vehicles. When it rains, the wipers spread this tacky film that smears your windshield.