This is because, in a monetary union of sovereign member states with a single monetary policy and largely autonomous economic and fiscal policies, government bond purchases are not a monetary policy instrument like our policy rates, for instance.
Government bond purchases are increasingly blurring the line between monetary and fiscal policy, and government funding costs are depending to an ever greater extent directly on monetary policy decisions. Euro-area central banks are now the largest creditors of their member states. All governments ultimately pay nearly the same interest rate on the debt in central banks’ balance sheets, regardless of the country’s creditworthiness.