What is the purpose of textbooks?
The question is based on several false premises and this also leads to an answer:


  […] students prefer to use notes which is[sic!] clean, short, and summarized


That's not true for many students. In fact, there are students who dislike summarized text and like textbook style.


  It is true that during an academic session, a student studies for a particular subject, but he may forget some of the material so he needs to study it again few days before the exam. Studying a 500 pages textbook few days before the exams is practically impossible […]


Textbooks have a table of contents and often an index, so it is not at all impossible to brush up on a certain topic using a 500 pages textbook last minute (given that you know the context and some buzzwords).


  […] you can look at the textbook for clarification but most students really do not do this.


Many students do this, really.


  If they struggle with a particular concept, they go to the office hours of the instructor and ask for clarification.


Some come to office hours, some don't. For my courses, only very little students come to office hours and most study by other means - presumably some read a textbook.


  The textbooks also have many exercises so if any student needs to do more exercises, they can uses the prescribed textbook but I have not seen any student who does this. 


Look harder. There are these students who do additional exercises.

Last point: Textbooks are used by professors to create their courses. There are great textbooks for introductory courses and professors do not want to reinvent the wheel (well, a worse wheel, actually). If they rely on textbooks for their preparation, why not tell the students which books they used?

I don't know if there are courses where you really have to purchase a textbook - you may just decide to not buy it and may be fine. I prefer to announce textbooks that are available online through subscriptions of my university and in the library as a paper copy (with at least one "permanent/reference copy" that can not be borrowed) so that the students can use the book for free.