It's based on net worth, not income. But net worth of billionaires is incredibly inflated by the way we measure it.
Bezos' net worth is based on the stock value of Amazon, and is based on taking the _highest_ price someone is willing to buy 1, _one_, share of Amazon and multiplying it by the amount of shares Bezos has.
No one is willing to pay 200 billion dollars for Bezos' shares, and saying they are worth 200 billion is simply false.
His net worth is dependent entirely on other people being willing to give him money for his Amazon shares, and that is dependent entirely on the value Amazon brings to its customers, or how willing people are to give Amazon money for Amazons services.
Now, if Bezos decided to sell all his shares for cash he could surely get billions for them, as Amazon is generating incredible value, but not 200 billion.