They are marketing to the schools and universities first and foremost because they are the ones who actually make the buying decisions. All the publishers try to do is make something that looks better their competitors and lets colleges cram more students into the classrooms. I don't ever remember them saying "students need this" but rather "universities want to see this". It is a broken system that relies on lazy/overworked teachers and massive hulking collegiate bureaucracies.
Good electronic textbooks won't come from companies that made shitty paper textbooks.
Some students mark up their textbooks. Others (like me) have a lot of books that will be valuable long after I graduate.
The problem isn't that we don't need new textbooks every year, it's that we don't need new editions every 3 years with updated numbers for the homework problems and a new picture on the front. That system invalidates used books that are in perfectly good shape, and should be abolished by professors who will stand up to the bookstore and let them know that they don't want the new edition.
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.