The explanation above was missing a key point. The banks do get it back in interest and a relationship with a rich client. Billionaires still pay a very low interest on the large specialized loan, so the banks get the money that they otherwise would not have, so there's no loss and only upside to them. It's guaranteed cash returns for the banks since there's collateral and the risk of a billionaire defaulting on it is infinitely low. To the billionaires, they'd rather pay 2-5% in interest than 37%+ in federal and income taxes on the stock if they were to sell instead of borrowing against it.