If money were created from thin air inflation would be out of control. Money would become worth less until a loaf of bread could cost $3000 and physical cash weighed on a scale rather than counted out.
Like Simon said, the banks are regulated by governments or at least they should be, and it is ultimately the government who is responsible. The interest rates, lending measures, inflation pressures, employment figures and so on, this is all considered by the reserve bank, keeping the economy well lubricated is their job. Printing money or creating it from thin air doesn’t make for a viable economy.