Musk (and most billionaires) don't have actual billions in the bank. Their net worth coms from owning the stock of the companies they founded. If the company goes bankrupt, they lose their wealth too. And paying for employees (10s of thousands of them) without any revenue or production can have an adverse impact.

If giant companies like Ford and Boeing are having trouble, it's not hard to imagine that a new car company that just recently crossed profitability is going to have solvency issues by staying closed.