Busy work

I find it pretty irritating that professor's assign basically books to read and expect us to have it done in about a week. Considering the amount of time undergraduates have (very little) and the amount of depth assigned readings bring to lectures or texts (little), I'll never understand why a professor expects me to have 200+ pages of a book read. If the reading is only glossed over in an exam, and hardly discussed in lecture, why should I read it? I'm loathe to read unless I know it will serve a purpose. I like to keep as enjoyable of an activity as I possibly can, and for the most part, reading for class is like getting your teeth pulled.