Written and visual storytelling are two different things, and something that works just fine in one medium may not work at all in the other. Imagine trying to do the classic music-plus-montage transition sequence beloved of film-makers everywhere with nothing but words on paper, for example. Writers being the creatures they are, some of them have probably tried it, and it’s possible one or two of them may have succeeded — but it’s bucking the odds.
Over on the written-to-visual adaptation end, you get all sorts of problems with adapting interior action — stuff that’s going on mostly inside the protagonist’s head — into an effective visual form. The key word there being “visual”; voice-over narration is not usually a good answer. In my opinion, any director who’s thinking about using voice-over narration should stop and think about it some more before going on with the project. The whole point of a movie is that it tells the story through visuals and action; throw in explicit first-person narration and you might as well have a radio play with illustrations. And that goes double for noir-detective-style first person. Stuff that reads on the page as moody and atmospheric and full of character-building through voice and tone tends to come off as purple and pretentious when spoken aloud. Especially when spoken aloud with pictures.
Sometimes, granted, Hollywood does make changes in written source material just because it can; but a lot of the time, the changes are made because something interior and/or verbal had to be translated into external action in a visual medium.