The loss is because they bought government bonds at high prices from bondholders to facilitate their replacement purchase of the new bond issue and thus monetize government deficit spending above tax revenue, causing a huge inflation, reaching 9 %, and the corresponding decline in asset/wealth value, needed to release real resources to pay for the government's consumption.
No private bank would lend you money in order that you could lose it in private spending.
The Fed does precisely this in order to accommodate government deficit spending, but the inflation is imposed on the citizenry by extracting resource payment in the form of real wealth reduction.
Because it is an indirect, sequential process, the Fed is able to hide it from general public comprehension, and pose as a White Knight who fights inflation by raising interest rates.
In effect they facilitate the government in paying its bills by printing demand deposit money.