Adapting almost anything from its original medium results in alteration and loss of quality. There is something very special I've noticed in reading (also in writing) and that is the ability of the text to "paint a picture" for you - or a feeling... things that generally do not translate well into non-text mediums.
Reading a book is a fairly personal experience, whereby you are imagining the sights, sounds, smells and sensations you are reading about and it is, more or less, a one-on-one dialogue between yourself and the author. Movies, on the other hand, are a mass-medium that present a blur of music and images meant to be shared by all who are watching. In short, some of the "personal" nature is lost, and perhaps that is why "a movie is never as good as the book."
Conversely, a game based on a movie is trying to create a personal experience from something that is a mass experience. Also, what was said earlier about mass-market franchise, a limited time to create the game and the programmers not really caring because the company knows it's going to make money already would also be factors.
That said, I've been VERY pleasantly surprised by the Narnia movies. I love those books immensely - they've been an influence in my own writing, so, naturally, when "The Lion, the Witch and the Wardrobe" came out, I was crossing my fingers and was so happy that it, and the later "Prince Caspian" turned out quite well. If they do go ahead and make "Voyage of the Dawn Treader" it had BETTER be fantastic because that was my favorite of the Narnia stories. I am a bit dissapointed that the movie makers used the "Bloodless Carnage" trope. I'm pretty sure I'd imagined more blood when I'd read the books. I was also baffled by the first castle siege in the "Prince Caspian" movie because I didn't remember that happening in the book, but it's been a while since I've read it, so my memory could be fuzzy. I was a little nitpicky on how the movie "aged up" Caspian, too... I'd always imagined him as a young boy, 12-13 or so, not as an Estrogen Brigade Bait young man.
Shortcomings aside, I was impressed by the addition of gryphons to the movies (I didn't remember them being in the books), and that... that is PURE LOVE for me, since I adore gryphons. That would be one thing that the movies added that I not only do not mind but am actually very pleased over.