In other words, we don't have to pay interest, we choose to, because inflation would almost certainly rise substantially if we printed a ton of money, and we're cautious (some might say overcautious) about inflation.

Of course there are some head-scratching rationalizations going around about why gargantuan deficits are good now, a decade into one of the longest economic expansions in living memory, but would have been bad to do when proposed in response to one of the deepest recessions in living memory.