They’ll say Batman is more relatable because he doesn’t have superpowers.

Okay, but he's a billionaire super-scientist who occasionally uses occult magicks. How does none of this qualify?

Superman grew up in small-town, working-class America

Sure, but how many modern day Americans could relate to growing up on a farm? Or getting a job in journalism?

The fact that he has the power to rule the world and chooses not to, is what makes him extraordinary.

I think superheroes are largely defined by their villains. And Lex Luthor - as an individual who regularly does struggle to dominate the world (and periodically succeeds with mixed results) - makes an excellent foil for this exact reason. Superman is, at his heart, just a guy trying to do the right thing. Luthor is an ego-maniacal fascist who cannot conceive of having less than total control.

The best Superman stories are ones that illustrate the practical limits of a seemingly omnipotent individual. It's Superman's struggles - his poor choices, his desire for human affection, his naive optimism, his inability to be everywhere at once - that make him relatable. The idea of Superman as a maximal human who still can't do everything has a way of taking the load of us, comparably weak and vulnerable people, who strive for just as much as a fictional demigod.