They can and do (although not quite in that way).

The main problem with it is that it devalues the currency. Money has no inherent value, so usually the effect of printing a load of money (or creating it on paper) is to decrease how much existing money is worth.

Simplifying a lot, the amount of actual worth or value stays the same (as nothing of actual has been created and no work has been done), but there are more dollars around, each dollar must be worth a bit less. And this leads to inflation which isn't necessarily a bad thing, but can be pretty disastrous if it gets out of control.