It may have something to do with the developmental stage MG kids are at. Younger readers, kindergarten and below, still believe in superheroes. They're just as real as Santa and the Toothfairy. First graders are starting to question. By MG age, they get that Batman is a cartoon character. So taking it too seriously would be like saying, I'm still a baby who believes in that stuff. But making fun of it--well, now, that allows them to still indulge in all the fun parts of superheroes without getting caught still wishing for it to be true.
