Central banks care a lot about money supply. They do not want to have to print money to account for a debt on the balance sheet, because that (eventually) devalues the currency they operate, and causes deflation which they regard as bad for the economy. They wish to use the money printer when they choose (and provide maximum effect for its valid use cases), not because they are forced. There is a money printing capacity they do not wish to exhaust. If you printed money to buy a car, but it was only because you had to make a big trip and you had actually been intending to wait for the new model, then that’s a suboptimal use of your money printer.

So basically the money printer is not free. There is an opportunity loss that they’ve found a way to measure as a real loss to make it easier.