They are protected by US corporate law. The debts of a corporation are the responsibility of the corporation. A corporation (Inc.) is treated like a person from that perspective. If a corporation runs out of money and cannot pay its debts and declares bankruptcy, the management is under no obligation to cover the debt, and the debt holders are out of luck.

A CEO (and other company execs) can be held personally liable only if there is fraud or other illegal acts. Mismanaging a company, or even simply working hard and not succeeding, are not illegal.

American firms can take on much more risk, so we tend to see both many more failures and many more successes.