And much of the real value of modern billionaires is measured in their credit-worthiness rather than their real liquidity. Elon Musk is a great example - a guy whose billionaire status is almost entirely bound up in how his car company and his social media company and his aerospace company are valued.

This describes most figures with a net worth over a few million - most of the time most of that wealth is in the form of non-liquid assets like owning pieces of companies or real estate. Much of their wealth is in their ability to borrow against speculative income from those sources.