They do, it is called quantitative easing. As long as you don't flood the market with the printed bills, the dollar value and inflation can be kept in check. But most countries is severally limited in their ability to do this as their currency value is usually measured against the US dollar as it is the reserve currency of the world. That is why national debt in the US is not a huge deal as they can always just print more money over time and get away with it cause of their reserve currency status.