"Why not?" you may ask. "Superman's an alien being! He's from another planet! He visits an Earth that is clearly analogous to our own!" Yes, all valid points. But none of these things make the film true science fiction.
First, examine the setting in which Superman takes place. It is indeed an Earth very much like our own; it's got a Grand Canyon and a San Andreas Fault and a United States and all sorts of other Earth-y things. But it's also got a Metropolis. Not only is "Metropolis" not an actual city, but it's a city that was designed specifically to be an allegory for every major... metropolis in the United States (hence the name; they may as well have called it "Big City"). Furthermore, while Metropolis is located somewhere in the US, it isn't explicitly or implicitly located in any particular state. Metropolis is not an alternate reality, rather it's a mere metaphor for reality, lacking the depth and specificity of a fully fleshed-out world. The characters follow this same pattern: Lex Luthor is a villain, Superman is a hero, Lois Lane is a love interest. These are not actual people populating an actual reality; these are symbols of good and evil populating a fable.
