A good reason to not pay it off is like the fact that US is borrowing money right now below the inflation rate - people are paying US to keep their money.

Also government debt is rarely paid off - it is usually grown out of and revolved. What US lack is economic growth thanks the "serious" people that push for austerity.

Also rule of thumb - anyone that uses a country budget is like the family budget analogy is - liar, dumb or ignorant. That is very quick filter by which you could judge politicians.

Because nobody wins. Having the world dependent on US stability is a good thing.