A web page can do the exact same thing except it is far cheaper or outright free, and it can be updated and deployed to people instantly. In many fields a textbook can become outdated in literally just a few months, and now you wasted your money. But professors don't care because they're usually making money off of them.

Textbooks drain the already strained finances of countless students and are found to give old or flat out wrong information all the time. There is literally nothing a textbook does that an online alternative cannot, it's only negatives.