Superhero comics are American mythology. They have their roots in demigods and heroes of the Greek/Roman tradition, and earlier traditions. Batman is a man of cunning, like Odysseus; Superman is super strong and fast, able to bound impossible distances, like Hercules, Gilgamesh, or Cúchulainn. We can even look back to American folk heroes as a precedent.
I'm going to use DC Comics here for examples because 1) I’m not as familiar with Marvel, and 2) I think that DC has the more iconic superheroes, in the tradition of fantasy/myth (but that's another rant for another time).
We have our gods/demigods (Superman, Wonder Woman) and our ordinary mortal heroes (Batman). They each have an origin story, which is similar to stories of heroes that you see across cultures, and a narrative path they follow. Bruce Wayne's parents are killed in an alleyway, so he takes up a crusade against crime, to stop that happening to anybody else. Superman comes from another world (in a sense his divine origin) and gets raised by ordinary parents (similar to the early life stories you see with semi-divine heroes across mythologies). Superman and Batman then protect a certain area (Metropolis, Gotham) from those that would seek to destroy it. Almost like a one-man Knights of the Round Table, protecting Britain from Saxon invaders.
And like the Arthurian tradition, the respective Round Tables see more members join. In the spirit of many older mythologies, it’s a younger generation. Just as the demigod heroes replaced the Olympians, who replaced the Titans, or as Fionn mac Cumhaill replaced the Ulstermen, who replaced the Tuath Dé Danann as the central figures in the culture’s stories, the Batfamily, Teen Titans, Supergirl, Impulse, and more begin to replace the older generation.
However. The commercial nature of superhero comics ultimately limits them from becoming true mythology. Batman can never retire. No Robin will ever be his permanent successor. Wonder Woman never dies. Nobody in the comics universe can age or die, at least not permanently. The characters never can grow and change too far from their original conception, because they won't be marketable anymore. At the same time, people get bored with the originals after awhile and leave. So comics are stuck between making the characters interesting and making sure they never get too far from the original, in order to keep people buying the comics. What they end up doing is endless reboots, retellings of the same stories again and again and again; not because they appeal to people on a deep level, but because they're banking on nostalgia, familiarity, and superficial curiosity.
It would be like if the Greeks endlessly told the story of the Trojan War, for a hundred years, only we're permanently stuck in the Iliad, Hector kills Patroclus and Achilles kills Hector only for all three to resurrect five seconds later with no lasting harm done, the gods occasionally get rotated in and out as interest waxes/wanes, and the quality of the writing gets worse every time. In the short term, it might get the publishers some money, with people buying the new stories because of nostalgia, familiarity, and/or curiosity, but ultimately it'll kill off the entire industry.
When my grandmother was young, she remembers every kid bought comic books and traded them, boys and girls. Today, comics aim themselves at a very specific age range, banking on the nostalgia factor. Or at specific groups, in the hope that a brand new character will draw in new readers—only to turn around and cancel that series, because very little depth was given to the character beyond “target X audience,” and people aren’t interested after the initial few issues. Comic companies might want to write a new female superhero (or revamp a pre-existing one), but if the extent of her character is “she’s a woman and that’s super rad,” most...people aren’t going to be interested. If she has no flaws, no arc to go through, nothing that makes her an interesting character, then she’s not going to draw in new readers and she’ll push away old readers. Every hero has some kind of arc or journey he or she undertakes, all the way back to Gilgamesh. And like I said above, this limits comic companies—they can never allow the mythology to naturally progress and change. It always has to be about what garners the most attention, the most profit—not about what the natural progression of the story might be.
This is why in fandom, where there isn't a need to make money, things progress more naturally. Go back far enough on Archive of Our Own and Fanfiction.net, and most of the stories in the comics category are about the original founders of the Justice League—Batman, Wonder Woman, Superman, and so on. Look there today, and most of the stories focus on or significantly include the younger generation. This is a trend that naturally happens with mythologies, and will most likely keep happening in fandom, if not in the actual comics.
tl;dr: American superheroes are America's mythology, but because of their corporate nature, they can't fully progress as most natural mythologies/legends do
