First, much of the Middle East is still a cash-based society. People actually do go out and buy houses or cars with shopping bags full of cash, dinar, riyals, pounds, whatever.

Second, much of the really large cash customers don't keep their cash in local banks. Instead, it is held off-shore, for better or worse.

Well, third... the rich Gulf States don't have anything like income tax and therefore have no way of knowing how much anyone is making, earning, giving away, investing, or otherwise doing with it.