A rich man will never say how much money he has.
But these guys said and why? they want to be called "the richest in the world". they want to get into history, just like the scientists, politicians, philosophers, astronauts who recorded their names in history, Adolf Hitler entered history in an evil way, Isaac Newton entered history in a good way. But these days there are few ways to get into history and they are using their money to get into the history
Being rich also kind of achievement and their names will be recorded in the history but the person who not earned all his money in the legal way can't express himself as the rich person because he will face the legal consequences for not having appropriate source for that income so the one who have all the documents for his earning is Jeff Bezos and he earned this by paying right taxes to the governments.