It's possible that you are not looking at the national debt accurately. When you deposit funds at a bank the bank is essentially borrowing your money, and paying you interest for the privilege. You lend (deposit) them money because you don't want to stuff it in your mattress. You trust the bank to pay that money back on demand, or if you have deposited it in an instrument like a money market fund or a CD you limit your ability to withdraw it in exchange for a potential higher interest rate.

In similar fashion the U.S. is now functioning as the globe's biggest bank, with no problem attracting deposits even with ridiculously low interest rates because it is by far the most secure game "in town". That could change of course, but looking at Japan as an example, their debt is now 250% of their GDP and they still struggle not with inflation but with deflation. Like us they owe the majority of their debt to themselves in their own currency.