That being the case, they often pump out "new editions" of books that have little noticible changes from the previous edition, except perhaps aesthetics and a different cover. That isn’t to say that there isn’t a need to update text books at all in this ever-changing world. It’s just that there isn’t a need to do it every single year and charge a student to buy another book all over again. With certain courses that change with society and technology, it is a necessary evil (computer courses would be the best example). But is it really necessary to trot out "new" editions of Math, when most of the principles have been in place for hundreds or even thousands of years? Chemistry, physics, many other sciences as well. Yes, technology does have a bearing on them, but not enough to warrant an updated text every single year. The only reason this is done is to make the school and the authors gobs of money. As I said, they are businesses like any other, and as such, are in business partially to educate, but more to turn a profit.
However, it isn’t simply for sinister profit that college books are so expensive. College textbooks are very well made by very reputable publishing firms, with high quality paper and printing so that they can stand the test of time. This kind of quality, in any product, never comes cheap. More and more textbooks are featuring bonus material such as CD-Roms and even interactive platforms, and this technology also does not come cheap. As well, they are written by experts in their field that devote hundreds and even thousands of hours of labour to a single text. Being experts, they command a high amount of respect as well as monetary compensation for lending such expertise to a usually highly specified subject. Quality labour never comes cheaply, either.
There is more to not re-using the books than profit as well. In high school, every student has to be there by law, not by choice, and therefore there is a standardized version of what every student will learn and which texts they will learn from, paid for by the taxes of the citizens. In college, it is the students choice to attend and pay tuition, and they are there to get the best education they can in their respective field. This being the case, the college must offer the student the chance to purchase the book rather than forcing them to give it back for re-use, as they are already paying substantial tuition costs and deserve the right to keep the material for future reference. As I mentioned above, it isn’t necessary to trot out all the new editions they do, and this is often done simply for profit. But to re-use the same text in a highly concentrated and rapidly changing field like Biochemical Engineering year after year would be doing a disservice to them and the good money they are already paying.