In superhero universes, it seems like good people with superpowers become irresistibly drawn to become super-crime-fighters of some sort, rather than doing all the other stuff you might be able to do with superpowers. Even when they seem to be otherwise inclined, it's the super-crook-fighting that actually seems to matter.

It's always seemed implausible to me... but they do give reasons: the origin story of Spider-Man shows what happens if you try to opt out. Spidey really didn't initially want to be a superhero, he had enough problems already, but he slacked off and Uncle Ben bought it.

I suppose the real reasons come from history. Superman and Batman arose at a time when people were really upset about organized crime, and Marvel Comics rose to dominance during a period when street crime was exploding (Spider-Man's first appearance was surprisingly early in that era, but the increase was already worrying people). In those circumstances super-crimefighters are a thing you really want to fantasize about.