They can´t. Simple as it is.
Most economies that are in real trouble didn´t get there over night. But all failures have a background. In the case of the US, this is continuous overestimation of own abilities due to its sheer size. When economic problems started in the 70ties, almost half of world GDP was American made.
But at that time the US economy was already sick. The cancer of loosing the competitive edge already started to grow, only very few people noticed because the patient, the US economy looked too big and too healthy. One victorious war (cold war) and a couple of desolate wars later, a dot.com and a housing bubble later the relative size of the US economy has shrunk to less than 25% of world GDP. But what is worse, by now the American patient is no more able to feed himself. The patient is in sick bay and even to keep the bed he lying on from falling apart, the friendly world nurse is bringing in own supplies to keep well admired and formerly proud and healthy patient alive. All the patient can do is hand out some fake money (bonds) to the friendly world nurse. While the world nurse is building her own new and successful economy, the sick patient US can´t even contribute and participate in this success story. The patient produces too little of what the successful nurse needs. Patient US can´t keep the own bed in sick bay in order, how can patient US provide new beds for the nurses economies?
Sad story, i hope you could follow. There are two ways to recover, to cut out the cancer of loss of competitive edge: Devalue the own currency through inflation, by printing money out of thin air (the American way) or reduce the size of the economy so it harmonizes with productivity (internal devaluation, the Greek way). Neither way holds quick recovery, but the latter, the Greek way is more honest, even though more painful.