I've noticed this in the larger sets and they probably could make a single book for maany of these sets. However, I think they do it this way to break up the sets into logical completion steps such as a single vehicle or scene. I think it's a way to keep younger kids interest in building to give a sense of achievement. Also it's a good parent's stopping point on many of the sets in that my son will build some of the larger sets and state "just let me finish this one thing." without taking three or four hours.
also there's a standard set of book sizes to choose from to reduce complexity at the printer. The instruction designers have to pick the format that the set fits the most efficiently into because they only have a couple of choices, but this saves money at the end of the day because you don't have to change out the printing machine all the time for different formats.