Because back in the day, the idea that grown-assed men would still be reading comic books and dressing like 10 year old's was unthinkable. They naturally assumed that these comic books would be read only by kids. They also assumed that these kids would grow up and get jobs, putting away childish things like comic books. They never could have imagined multiple generations of grown ass men slacking their way through life while wearing images of their childhood comic book heroes on every t-shirt they owned. These comic book franchises make a fortune because there are millions of adults who live in a state of arrested adolescence the likes of which a young Stan Lee could not have predicted.