How many people do you think there are? You'd have to a) control vast resources or businesses, b) make sure no legit journalist in the entire world can tie you to it, and c) not care about achieving higher returns by investing in legitimate options, d) grow to that fortune without using mainstream investment options, and e) restrain your spending enough that you don't arouse suspicion. Not to mention that a huge, illicit fortune would be worth less than face value because it would be less liquid than a legitimate fortune.
Let's look at the top 3:
Buffet: started one of the most successful investment funds ever, which now owns most of Coca Cola and many insurance companies, among other giant, prominent copmanies
Helu: owns most of the telecom in Central and South America
Gates: started and ran the largest software company in the world, running on ~1B computers. Was one of the biggest business and technology success stories of the century.
You have to be prominent to get those big numbers.