Why did Congress design the Fed this way? There are two very important reasons. First, monetary policy affects the economy with sometimes long and variable lags, but elected politicians, and even the public, often make decisions with the next month, next quarter, or the next election in mind.
Monetary policy actions taken today will often not have their full effect on the economy for at least several quarters and perhaps as long as several years. That is why monetary policy choices today must focus on the intermediate to long term and anticipate what the economy might look like over the next one to three years.