Nah, almost never. That is usually forced by the university, publishers and, surprisingly, by the students and their parents:
Say the textbook used for the last 5 years costs $300 and now you want to switch to a different, cheaper book. Now you have to convince admin (why switch? The previous one works) and students (I pay $$$ tuition, why are you cheaping out on the book? Is it worse?).
> how much royalty do they get?
Basically nothing, so that is obviously not the reason and you should be ashamed of yourself for suggesting that.
Say the textbook used for the last 5 years costs $300 and now you want to switch to a different, cheaper book. Now you have to convince admin (why switch? The previous one works) and students (I pay $$$ tuition, why are you cheaping out on the book? Is it worse?).
> how much royalty do they get?
Basically nothing, so that is obviously not the reason and you should be ashamed of yourself for suggesting that.