Maybe that's been your experience, but over here it is a big money-maker. I had a paperback last semester (http://www.amazon.com/Frauds-Myths-M.../dp/076742722X
) which cost over $50 and was written by a friend of the professor teaching the course. It was a good book, was well-written, and served its purpose. It also would cost about $10.95 if it wasn't a "textbook".
I would also wonder why someone would write a book and not get paid for it.
I will agree with you that the publishers and college bookstores are in on the deal. I should have mentioned them as well. My experience last semester (and others like it) made me think of professors when the subject came up. I am also sure that the majority of professors work very hard for any money they may get from writing a book.