Well, I don't think he's an idiot, at all.
Here's what I think happens: A lot of filmmakers love his ability to write for grand orchestra in a dynamic, post-romantic style, and not only request that sound when meeting with him, have already temp tracked their film with his previous scores, and fallen in love with it. These same filmmakers don't have the same discerning ear that composers, or most studious fans have.
Then, it gets time to scoring, and deadlines approach and Horner focuses little on originality, and everything on using his formula to match perfectly to the picture, to exquisite detail. He hits the deadline, appeases the filmmaker, 99% of the people don't notice any self-plagiarism, and most of those that might, don't much care.
So, I think it's an issue with being passive, or lazy with his compositions. To me, I wouldn't mind so much if he just said so. Or when prompted, say that he's doing what the filmmakers like, he likes his music sound, and if you don't like it that's your opinion, too bad. But to act like it isn't so and everything is completely original is nonsense.
Strangely, his ability to write very precise orchestral arrangements is what he's most praised for, but also where I find his most repetitive work. It's when he reaches out into experimental, ethnic and electronic stuff he can create some diverse, and at least mostly original works (Flightplan, Iris, House of Sand and Fog, The Forgotten, Beyond Borders, The Missing, etc.), works that are sometimes criticized for lacking the detail and depth (counterpoint, etc.) of his orchestral works.