Why is there such an extended period of time between a Presidential election and when the President-elect takes office? It seems to me this time period allows for all kinds of shenanigans -- from shredding documents, pushing through judical appointments for the losing side, and having Senate leadership positions filled before new senators arrive to take their positions.

How did this practice of a large gap of time get started? Who started it? Is it an actual legal thing or just a convention?