I had one prof here in Germany that did this as well. It was his own book. He was adamant that everyone taking his class had to get the latest version, upgrading it every semester.

I'm a lecturer for a University where all courses are required to have a mandatory textbook that is sold at the University book store.

The thing is, most of the time the book isn't actually necessary - we just have to tell the students in code. When I tell students that a textbook is mandatory as a required supplement to the materials that will be provided in lecture and online, it's code for "Don't buy that shit - they're making me say it's mandatory."

When I was in school, I also had a professor who had written a mandatory textbook, but it was printed on the cheap, spiral-bound, and he didn't take a commission on its sale, so it was like $12.

But there are also evil professors. I had a professor of American History in college who had a "mandatory" textbook he had written and said would be included in all the exams and that he would not cover it in lectures. The book turned out to be about an art movement in 1920s Mexico (his doctorate was in Mexican history), and not a page of it was actually relevant to the course or included in the exams. He just wanted to make extra money off the students.

in k-12, its usually based one book, that almost never changes edition, or very little, and the schools doesnt put money towards newer updated material.

most of the classes require questions answered specific from the book like most hw, or whatever questions derived from that specific book particularly. thats why its hard to get by without a book.

when the info is all over the internet, or in random books, students might not be able to connect all that info together. most students need some kind of info all in one book to learn and anything unclear then they look at online resources or other books. maybe your countries universities present thier lectures differently.

but when professors lecture with only slides, people rarely learn from them, because they are very basic and generic and often unhelpful for the most part, its almost exclusively done by professors that cant be bothered with teaching a class, rather do research instead.(not all but most of them are like this).

plus new books/classes often come with a required online component, where you get a special purchased code to use, and its part of your overall grading system in the class.

it maybe EU/germany professors lecture slides differently than the us. the prof here just do it lazily while barely explaining the material, and already expected the students to know the subjects.