It's actually normal, if you look at the top 20 richest people, they're never in that top 20 for long.
An it's not as if he ever ha that much money in the bank.
When you own a lot of shares in 1 company, trying to cash them in would usually cause a rapid drop in their value.
If I had a billion $$$s in the bank, I wouldn't even bother investing in anything.