Well, then he will make a marriage with a mere mortal? Then all their wealth during the divorce will be destroyed. So they marry only with their own in order not to lose their fortune 
I don't understand why these billionaires refrain from pre-nup arrangements before their marriage. Nowadays divorces are extremely expensive, and can ruin someone completely. I am not just talking about Tiger Woods BTW.... Look at Dmitry Rybolovlev. He is about to lose somewhere around $6 billion.