The Capital Gains Tax is only 15% where as the tax for the highest incomes is 35%.
You can make a killing of millions by playing the stock market and pay less than half as many taxes as if you made millions through a salary. That is why insurance companies, many healthcare companies, companies with angel investors and venture capitalists, and companies on the stock market are for profit. They want the opportunity to make a lot of money and you can make more money (that is, get taxed less so you take home more money) by owning part of a for profit company that being a founder of a non-profit company.
You don't get to be a billionaire by being on salary. You get there by owning part of a company who's value skyrockets. Warren Buffet, professional investor; Bill Gates, Microsoft founder; Herb Kohler Jr., Kohler Owner; Mark Zuckerburg, Facebook and the list continues. You can get rich working for the man, but your wealth will still be determined by the man.