Also, someone like Mark Zuckerberg has almost 0 default risk. To the banks, this is free money. Remember, when the bank issues a mortgage for 3.5%, the spread

Also, someone like Mark Zuckerberg has almost 0 default risk. To the banks, this is free money. Remember, when the bank issues a mortgage for 3.5%, the spread between that and their cost of capital covers the risk of default. But if the default risk is minimal, they can basically issue the loan at cost.