The problem is this: once you have a high level of debt, you become sensitive to interest rate changes. Your debt rises, so you have to increase interest rates to lure investors to buy the bonds. The high interest rate forces to you run a bigger deficit, do you have to borrow even more money. Then investors demand an even high risk premium. That's the cycle.
Once that process takes hold, the only way out is some combination of lower living standards, higher taxes, higher inflation and currency devaluation.
You're right that it hasn't happened yet in the modern era. But it will. Remember where you heard it first.
I first heard it in a book written in 1986. It showed debt growing exponentially and forecast hyperinflation and economic collapse. People have been crying about that particular wolf for quite a long time.
Historically, governments have been prone to currency crises when their debt-to-GDP ratio exceeds 100%. The USA is just slightly above that now, and interest rates have remained low here (and in other countries) because central banks are monetizing the debt. But that can't be sustained forever.