But they can relate to Batman and Spidey?
Yes. Superman is an alien with godlike powers who is raised as an all-American farm boy. Neither of those are relatable to the vast majority of the world's people.
Spider-man was a nerd underdog/outcast trying to juggle a relatively normal life of a young person without much money with powers that were an accident. He also fulfills every powerless person's fantasies of being given great power, aside from dealing with everyday relatable problems.
Batman is a regular human being and an orphan whose iconic imagery is based on overcoming fear and trauma/loss and turning them against themselves. Those themes are much more primal than anything in Superman.
Even if you don't buy those arguments, though- the fact remains. Spider-man and Batman are easily the two most iconic and popular superheroes worldwide. My friend's mom is an immigrant from India, and she can sing the old Spider-man theme song even though she isn't a superhero fan, but she doesn't know who Wolverine or Green Lantern are. My other friend's parents are from Cuba, and they love Batman but don't give a fukk about other characters. The same has proved true everywhere I've traveled so far.