Because it is an additional expense and fairly often you don't actually need it. You are paying a lot of money for that one chapter you will maybe have to read and whose content you can likely find elsewhere.

Plus, the idea that everyone should buy it is bonkers. There are or should be libraries in school that costs $10k a year. Or at least, there should be used book from last year or rhat one book 5 friends bought together. All these would be financially reasonable decisions.