I posted the question on a political forum that I belong to, and they stated that in the 2-1/2 months between the election and the inauguration, the president-elect (unless he is the incumbent) has to set up staff, become briefed, manage all sorts of political issues that he will have to attend to when he is president ... etc. etc., and that all of these items take a lot of time.

It seemed dubious to me, but many people responded that way when I posted the query. I don't know why a president-elect-in-waiting couldn't proceed on the basis of expectations, however.