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

It is related to the general definition of inflation.

Those with existing assets (including retired folks in the US) theoretically "lose" in this situation.

You can split the US and see inequality arise from printing: (1) US federal govt vs non-govt entities (2) US citizens with assets vs US citizens without assets, (4) people who received money first vs people who last bits of the "trickle down".

In various cases, certain people "win" and others "lose"

So the current system of loans/ bonds generally benefits the rich (or people with assests)?

Because "borrowing" money 1.) recycles foreign dollars back into the US economy 2.) maintains foreign interest in accepting US dollars for goods and thus sustains US trade deficit 3.) prevents idle foreign US dollars that would other wise buying US assets that then inflates their price (eg real estate)