I had always thought that was the case as well. But then when I mentioned this to someone (current college student) she told me than in some cases it was because soon after the new edition appears, all of the problems can be found, worked out in detail, on the web.
Well, there's always that.
The publishers were well on their way with this scheme way back when I went to college when "internet" was the space between two commercial fishing boats. Of course, students still passed notes the old-fashioned way.
That is amazing. My Hegel theme party was a bust.
If you click on my spoiler in the post immediately preceding this one, you will see that my Heigl Party was a bust too.