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.

German execs are also insulated from the debts of the company. Many German startups even use English (now Irish) limited companies.

I think you answered your own question. In the US, entrepreneurs are not personally liable. Now, that's not the same thing as saying that failure won't reflect badly on their reputation. But it is definitely a lot easier to have a comeback career if you don't have to declare personal bankruptcy.

Consider that perhaps new enterprises would not have as high of a failure rate.

Mostly because there are fewer new enterprises. People mostly only take the safest bets.

They move to the United States and take their companies there.

That's part of why Germany has fewer entrepreneurs.