1. Provides a chance to get outstanding political work from the Session done.
2. It takes a couple weeks minimum for governments to have certified election results. If you're in California, triple that. So the absolute earliest you could seat a new Congress and president would be around the time the Electoral College takes place, mid-December.
If you read my post, the California Legislature takes office on the first Monday in December. There is no reason why the United States Congress cannot take office at least that soon after the election. I would agree that the transition period for a new Presidency (although not a reelected President) takes a bit longer than assembling a new Congress.