Sweating is a property of humans, animals don't sweat, and we know Superman is extraterrestrial, so he must have some other way of cooling, that's it. You would

Sweating is a property of humans, animals don't sweat, and we know Superman is extraterrestrial, so he must have some other way of cooling, that's it. You wouldn't sweat if you could cool down by other means. Skins, including those of some animals, also produce an oily film that resembles sweat.