eventually they presumably become too worn down to have resale value,

Or 2 years later, the lecturer/tutor and publisher release a trivially updated version with everything shuffled around, making it unfit to follow along with the lecturer's trivially updated/shuffled course plan. (and of course most other teaching institutions follow, because it'd get complicated to have students try to reliably source one non-current revision). University textbooks are a high-value cut of this resale system, yet a niche quite apart from the rest of the industry.