1. He Has No Enemies Who Are Worthy Of Him
The above mentioned Lex Luthor and Zod are easily Superman's most famous enemies and also infinitely weaker than he is. The idea of having the villain being weaker than the hero is a great one but not if the don't have a legitimate way to challenge the hero. For example, just about every villain that The Hulk ever faced paled in comparison to him but it was the nature of Bruce Banner as a character that kept those villains compelling. Banner was like a ticking time bomb whenever he wasn't The Hulk so in reality he was his own villain. Superman is not only the most overpowered being in existence but Lex Luthor is literally a normal, human man who has a lot of money and resources. His evil plans always involve destroying something major and when Superman arrives to thwart him there is always Kryptonite. Lex Luthor may have the brains to back up the brawn that he lacks but it doesn't matter because Superman has both. This all goes full circle because it reinforces my first point about Superman just being too powerful to make him an interesting superhero.
Thanks so much for reading. I'd love to hear your thoughts and opinions in the comments.