Why Superman Doesn't Take Over The World

What do superheroes have to do with the economics? 

In Professor Brian O'Roark's class, he uses superheroes to teach economics and to present concepts such as scarcity, resource allocation, game theory, and the invisible hand and has written the book, Why Superman Doesn't Take Over The World.

For those who have their favorites ranging from Wonder Woman to Spider-Man (or anything else in the Marvel Universe and beyond), have you ever thought why do superheroes not monetize on their powers and most importantly, why do they even bother with day jobs:

You would think that one of the perks of being a superhero is you wouldn't have to go to work a 9-to-5 job. But there is a lot of these characters who - they go out and they get a job because of the type of service that they're providing.Essentially, they're providing public goods. And because they're providing those public goods, they can't charge for them.

If the world is in constant danger, perhaps, as a superhero, you cannot think of making money if there is no world to live in. But, the other interesting thing is the amount of faith that superheroes have in their governments and legal systems (usually) because why not take over the world yourself? Also, if they are working for the public, should they be considered federal employees and be paid a stipend? 

Maybe, just doing good, is enough.