I think they expect most games to start at level 3 (or at least hit it after 2 sessions). Starting at level 3 allows the players to start off as heroes, rather than a common person, which is a common theme. Starting at level 1 allows the zero-to-hero approach, which is also a popular theme.
However, I feel that WotC missed an opportunity here. If instead of front-loading most of the classes, they spread them out over the first three levels, and assumed that most games started at level 3, multi-classing would work a lot better. Instead of taking a single level of a class to get most of what you want, you had to take 2-3 levels, it would discourage the "level dip" that is both prevalent and disliked by many DMs.