I've personally seen new editions feature nothing but shuffling of the contents. They've even started selling "loose-leaf" editions, meaning that it's just a collection of binder-punched pages. Why? No new textbooks = no used textbooks = no competition from third party vendors selling used textbooks. No one wants to buy a bunch of pages in a binder.
Seems to me that an open-source textbook should be written once and updated incrementally as needed. Up-to-date information could be available online for those with slightly dated textbooks. Of course, the universities, McGraw Hill, Chegg, Amazon, and numberless other interests wouldn't gain much from this situation.