No, new editions are churned out every 12 months or so; of course it depends on the course. I remember my strategic management book was in its 14th edition, and its first editions was only in the 90s.

And the section numbers, case studies and questions get shuffled around too, so the previous edition is not sufficient.

The market is inefficient because the folks who choose which books to buy - the people who set the course - are not the people who have to pony up the costs. So it's in the interest of the publishers to maximize their gouging of students, and then spending a good fraction of the surplus on "incentives" for course setters to keep them on board and recommending the latest edition. The publisher who gouges the most has the most money to spend capturing these interests, creating a feedback loop, so for any given course vertical, there's a tendency to a monopoly.