I'm not sure that college textbooks are written once. It depends on the subject, but they are often replaced frequently e.g. as technology or laws are updated,

I'm not sure that college textbooks are written once. It depends on the subject, but they are often replaced frequently e.g. as technology or laws are updated, and each one makes only a small number of sales. Yes, printing costs are relatively low, but they are also a relatively small part of the cost of many kinds of book.