My understanding is that in the 90s and 2000s publishers started realizing that they could sell bare bones textbooks and then sell those problem examples in compendiums, effectively double dipping on each student.

Edit: For example, books like this: https://www.amazon.com/-/es/Gregory-Grant/dp/B08FP7SNZJ

> In addition to well-explained solutions, this manual includes corrections and clarifications to the classic textbook Linear Algebra, second edition, by Kenneth Hoffman and Ray Kunze.

I think this is actually fair; even Strang, after all, provides answers to only select problem sets (e.g. only odd problem sets in Linear Algebra if I recall correctly) though to be fair to Strang, it seems to me that in his case, it's more a matter of "if you are unsure, come and ask me" and probably not the case of maximizing profits.

This so much but in mechanical engineering. I was struggling in my course when I happened on a book written in 1980s in used book store for $1

It had worked examples, basic theory from foundation up, and was absent all superfluous info.

I credit it to passing my course and getting to where I am today. I still have it in my office to this day.