Do teachers generally refer to hard-to-comprehend books?

One possible reason is that I feel the style of textbooks have changed in the last years, so that newer books are sometimes easier to understand (use more modern-day language and symbols, is friendlier to the reader etc.) (This does not necessarily imply that they don't cover as much details as the older ones or are more superficial) But the professor, being older, does maybe not know about them or is used to the older style. And often, some old books are considered classics that all students should know regardless of whether something better came out in the recent years.

Additionally, another reason might be that your professors don't have any education in teaching and don't know much about it. Most of my professors thought the most effective way of teaching is to copy lecture notes onto the blackboard and read it out loud with a monotone voice, without any explanations or motivation because they assumed that having it written down means students know it. Those may also be the types of professors who recommend you a hard-to-read, but factually correct book.

Faculty (and people in general) do not have time to read multiple books that describe the same thing. So they will suggest the book they have read, not the best book.

Further, it is unlikely for two people to agree on which book is best.

Note that you and your teachers might have divergent goals. Their ideal goal is having you understand the topic, as this is what matters in the long run. Grades and exams are only a tool to this end. After all, if they want everyone to get good grades, they could just make the exam easier.

But especially for beginner classes there are often a bunch of books that drill the standard exam problems, without properly explaining much of the actual topic. They might seem helpful to you, but for the goal of teaching a topic they are actually counterproductive.

Furthermore there is also the less obvious to spot category of books that offer simple, intuitive explanations for most of the topics, which seem easier and helpful to you, but whose explanations just turn out to be too superficial or possibly even completely wrong. Most professors have experienced having to waste time helping their students unlearn such bad ideas, so they will be wary of any books that seem to easy.