It’s a debt to itself.
In ELI5 terms, if you have an allowance, it’s like asking your dad for an advance now to get a bike so you can make money delivering newspapers and return it back to him sometime later. You owe a “debt” to your dad and you’re working hard to pay it back, so your dad is probably never gonna call it in.
National debt is a good measure to know how much of an advance you’ve taken and what you’ve done with it so far.
The same reason nobody worries too much if a person that makes $500,000 a year takes out a loan for a million dollars, but someone that makes $40,000 a year can’t get a loan for more then $200,000 or so.
Rich countries have lots of money and can barrow lots of money without it being a serious problem.
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