We pay it back all the time. Our debt exists in the form of Treasury bills and bonds, which are dutifully paid when they mature (anywhere from a month to 30 years later).

The total debt goes up because we keep issuing new bonds. But bondholders are satisfied that we do pay back our debts, so they're willing to loan us more money. They're so confident that we get the lowest interest rates; every other interest rate is pegged to that.

It's important not to be misled by analogy to personal debt. You will die some day; countries do not. If somebody loans you money they have to take into account if you will die before you pay it off. That's not a problem for the country. (Or a company; many corporations also have permanently revolving debt.)

Now, if we ever fail to pay back a debt, that entire house of cards collapses very, very quickly. We're currently at no risk of that; we make enough money to pay our debts as they come due. But a government could decide to fix the "debt problem" by failing to pay them, and that would be so unbelievably bad that it's hard to imagine why politicians even suggest it.