They're not. They rely on visuals and music to enhance the story and the reader can interact with and change the story.
That's not to say they're inferior to books, although I wish the medium would explore something besides the harem and fantasy genres
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Anonymous2012-01-22 19:38
Meh, it's a mixed medium. I suppose it would depend upon the quality of the work. Maus won a Pulitzer Prize if I remember right, and it was a visual novel.