If the united states economy collapses, it won't only affect america, the entire world economy will be brought down.
Yes.. that is the problem. The global markets are dependent upon the American markets (NYSE / NASDAQ). Almost every single bank in the world is interlinked to the American banks. Even the forex rates for the other fiat currencies and even Bitcoin are based on the United States Dollar. It is the world's reserve currency and the only widely acceptable currency for international trade.