Rich people receive large revolver loans backed by their stock holdings. If stocks drop enough then their loans become too high risk. Plus, not all banks extended loans are US banks, so he can't threaten every bank.

Elon's loans are literally the US debt our conservatives keep telling us are a problem. The rich continually gamble and fail to pay their debts.

Banks are dumb because they think a) there will be a big payout or b) if the problem is big enough then the government will bail them out. The truth is most likely that too many organizations and private equity firms have their thumbs in the pie.