Why aren't open market operations a zero sum transaction?
When we talk about the money supply, we are often referring to credit that is circulating in the economy at a moment in time. In other words, the more assets the Federal Reserve takes on, the more the money supply will increase at that moment. Indeed if the Federal Reserve decided to reduce its balance sheet by letting treasuries mature and the US treasury absorbed an equivalent amount in taxes to pay off its debt, the money supply will shrink back to where it started before the government issued those treasuries. But that will be far into the future if it even ever happens, and so for the present time open market operations do nominally increase the money supply.