One part is because of that unique property : In capitalism, one’s revenue is another one’s expense.
National debt (debt of the government to its own citizen companies) mean the government give money to its citizen making its economy work.
A government that “make” money to pay back its debt would, by that first statement, be taking money away from its own citizen.
Debt to other countries is a bit different but this is ELI5 and the international economical system is not ELI5 compatible (some would say on purpose 😛)
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