Thomas Shey
Legend
Advancement is superhero fiction is so weird. Sometimes there’s an obvious skill or power increase over time while sometimes there’s nothing for years or decades, then a massive leap ahead. Or the perennial favorite of complete power swaps. It just doesn’t map to standard RPG expectations of advancement.
The one general exception is the teen-supers genre, where some visible advancement is almost always present over time. That's admittedly a special case, however, and your general point is sound.
(That said, you can make some of that argument for any persistent episodic fiction. Most of them the characters are at least semi-static, but it'd be unlikely most people would want that in an RPG based on it.)