Historically, your lazier instructor took problem sets out of these books which put extra pressure on students to buy them. There's also the accelerated edition turnover in the publishing industry, so that teachers always get the latest edition, which has slightly different problem sets than the one from two years ago, even if the material is the same as it was two decades ago. It's hard to feel much pity for any lost sales suffered by those outfits due to online distribution of current texts.
Today, any instructor with access to an LLM can come up with unique problem sets and solutions with relatively little effort for a whole semester's coursework, and just do that every time they teach the course. Yes students will just use LLMs to help them solve the LLM-generated questions - so more in-class quiz sessions are likely to become the norm.