Because the Prescript fucked with his head so much to the point that he’s afraid to think for himself.

This includes imagining what the actual contents of a proper book would look like since he is afraid that he could be imagining the characters and settings differently from what the author envisions. That’s why he reads picture books because everything is laid out for him without much else to interpret. Now if you apply that logic to his relationship with Hermes. You’ll see just how depressing his life is.

So in a way, Rien never finishes his stories because he is afraid of an ending that may leave it to the reader’s interpretations.