Many successful novels fall into that latter category. Sure, the films can be made, but for whatever reason, the films often fail to make that same connection with the viewer as the novel made with the reader.
There are those odd novels, here and there, that are so large in scope that not only can they not be made well, they cannot be made at all.
Peter Jackson‘s Lord of the Rings trilogy was a project that most thought “couldn’t be made.” At 1,000+ pages, Tolkien’s Lord of the Rings is actually a three volume set divided into six books and all contained in one novel. Jackson not only made the films, but he made a very fine set of films; worthy of the novel.
Stephen King’s The Dark Tower series is a seven volume set contained in seven separately published novels totaling over 3,000 pages. The series also ties in with stories from other King novels such as The Stand, ‘Salem’s Lot, Insomnia, The Talisman (by extension), Black House, Hearts in Atlantis, Eyes of the Dragon, as well as several short stories published in anthology sets. Granted that the references within the Dark Tower series to most of the other stories are more like tie-ins and shout outs to loyal readers than actually substantively part of each of those separate stories, but there are several novels with key components to the story, and the point of all of this is to paint the landscape that is the Dark Tower as being as immense as the tower itself.
Turning this series into film just can’t be done.