That does increase the supply of money.. The money used for that is newly created money, not money already on hand. It goes to the holders of those assets rather than government, but it is money (as currency) added to the economy. The difference (between QE and creating money for the central bank to use to purchase newly issued debt of the central government) is that the amount of debt in the economy is not being increased. (Technically, debt is not being decreased, either, though those assets can sit on the central bank's books indefinitely--or eventually be written off, if 'nonperforming'.)