Lots of times teachers are the author of the book, getting kickbacks and / or perks from the publishers, or are simply too lazy to care how atrocious the status quo is.
A lot of times the person spent so much time and energy writing their book, this is a major motivator to mandate / force it's usage whenever possible.
One time my art teacher made us buy his book, but it was only $30 and was pretty good actually, and I still have it on my shelf today. However, such cases are the exception.