It's not me who doesn't give the slides. Some people are just precious about their work. I personally don't think this type of attitude is great for a teaching professional.
A considerate professor would allow using an older or multiple editions of the books, and assign appropriate readings or problems for multiple editions. Libgen e-books are a strictly better product anyways: you don't have to carry it around and you can annotate them without the notes being inside the book.
Typically, the issue is slides allows the speaker to present stuff much faster than a person can realistically write (unlike writing on a board), so you end up with lossy notes. The coping mechanism for students is therefore by writing notes on the slides. Slides also help you preview the lecture, though few people probably actually do this.