Why haven't some indie developers made this?

Imagine a video-game almost identicle to pokemon, but with all your favourite superheroes and just about every superhero you can imagine.

You go around and make the superheroes duel, level them up, and then try to defeat super villains (and upon defeat you can recruit them).

because it would be a massive shit storm.

they already have, DCUO, Marvel Heroes, HeroUp.com etc...
There's also that facebook game Avengers Alliance which seems to have a pretty solid fanbase