They can't carry out but so much "cash" at one time.
It's obvious that you have not been around many wealthy people or taken any economics courses (that you passed), or you would know just how stupid that statement is.
To begin with, really wealthy people seldom do anything with cash. Most have taken the advice of the financial advisors, who generally suggest that if you have a lot of money, at least half of it should be invested offshore. When the wealthy leave, there are many very easy ways that their remaining wealth can be transferred offshore. In fact, some of the methods available can even be a legal write-off on their last income tax filing. Creating offshore debt is one of the more common ways to move money. If an offshore entity owes you 100 million dollars before you leave, that entity still owes you 100 million dollars after you settle in Bermuda, Ireland, Belize or the Caymans. The note comes due after you have settled elsewhere. Voila! Limitation avoided.
To do it all legally requires a bit more work, but there are indeed, many legal ways to move large amounts of money between countries. Wealthy individuals and corporations do it every day. Of course, Congress is busy trying to create new laws to limit how it can be done. But, it is those laws that are responsible for the capital flight that we have seen so far.
You even find that when they leave, the wealthy leave nothing of value behind. You will discover that their house is owned by a foreign company that has it mortgaged to the hilt at some foreign bank. If you want to buy the house, you, not the billionaire, will be the person that is sending the money offshore to pay the foreign owners. Other properties, such as factories and office buildings, will be similarly encumbered. If the US government should suddenly refuse to enforce those foreign debts, our international credit rating would collapse and our markets would go in the toilet over night. Even the greedy, power-hungry people that we have in Congress are not so stupid as to risk that.
In fact, you can be sure that if a billionaire wanted to move 20 billion dollars or so offshore, he could probably do so within 9 to 12 months - LEGALLY. And once it's gone, it will be a long time before the US sees any of it again. In fact, since many products that were once produced and sold in the US will be produced offshore, it will mean that more and more money will be going offshore as Americans continue to buy those items. It isn't a pretty picture.