Books have a much longer history of use, books are reliable and convenient. Some people cannot read from the screen at all, only paper books.
DVDs, like tapes, are not reliable in work, they were released much less, in relation to books, they do not have the romance, like vinyl. DVD requires special equipment that is difficult to obtain or repair. DVDs disappeared relatively recently, and did not have time to acquire collectible value.
There are many reasons, I will tell you about one. Choosing a book and buying, a person thereby presupposes rest and entertainment outside of gadgets, with which he is already in constant contact. In order not to sit in a boring backlit phone and resist the temptation to check notifications, but to sit with a paper book detached from the whole world - this will already be different from the main pastime and will give more fresh emotions.