Simply put: "You're not gonna earn a billion dollars pulling down regular income and paying regular income tax on it. Just not gonna happen."
FYI, Abigail Disney — the granddaughter of Roy Disney (that's Walt's older brother), whose net worth is "nine figures" — said that.
And there are "evil" billionaires. The billionaires mentioned here are from the western world. In some countries (i.e. UAE) billionaires treat their workers like slaves. I admire Elon Musk very much for what he achieved, it's almost superhuman but contribute to society by paying taxes!