For lots of reasons. It is in the interest of the US to lend money to other countries. It helps those countries. It improves relations between them. Most importantly, when the money is paid back, the US makes a profit.
Economics enables us to understand money, its behaviour , and importance. Economics makes a nation prosper, and makes a person educated in a real sense. Economics is defined as a science of money and money being the most important factor in modern man's life it is most important to study.