Taking out a loan and promising to repay it is a voluntary exchange of value that markets are comfortable with -- to issue debt is to play by the rules. Printing money directly is just an exercise in pissing on the users of that currency. People don't want to use currencies that do that. When people buy US debt, they are implicitly validating their trust in the currency to be paid back later.

Some other countries have simply tried to print their way out of deficits, usually it doesn't end well. Zimbabwe is my favorite example. https://en.wikipedia.org/wiki/Hyperinflation_in_Zimbabwe