The value of a dollar is partially set by how many dollars there are. If you just make more dollars out of thin air, the value of them is lessened and the value of having made more dollars goes with it.

Our money is fiat money. We literally do make money out of thin air in the first place. Fiat is roughly latin for the decree "let it be", or creating something without effort. For example, "God created Earth by fiat. He commanded it to be, so it just is".

It's a faith based system "backed" by the economy. The value of for example the US dollar is set by their GDP, and how much fiat currency there is proportional to it. In theory, assuming a static amount of demand you can print more money out of thin air as GDP rises and the value of the dollar will stay the same.

The inflation we know in the US isn't inflation from printing money. It's from pull-demand, or demand of goods. You can see here in this chart that the actual value of the US Dollar hasn't changed much over the past 40 years. It lowers and rises based on our economy (most notably you can see the 80s market boom in the value of the dollar, and the 2009 recession).

GDP grows by about 2-3% per year, and the fed prints about that much per year, plus some extra to account for destroyed currency; most they personally destroy, some based on estimates that are destroyed out in the wild.