Two basic reasons explain why movies about musicians and composers tend to be lame. The first is simply that a great deal gets made up to make things more interesting. Composers get lives that they didn't really live. Amadeus is one of the most flamboyant examples by giving us a Mozart that wasn't particularly Mozart. He's presented as a punk/rock wonderchild who didn't get a lot of coaching and advice from people like his father or Haydn. The second is that art itself is presented as a kind of savant's game, that there is nothing much to analyze about it or struggle with within in it and that the stuff just magically oozes out of artists/composers/poets et al.
The subsidiary problem in this second difficulty is that you don't really see people slogging through proofreading manuscripts, bantering with editors and publishers, filing paperwork, hiring a copyist, slogging through decades of work on what might be a ten minute piece of music. Three to ten minutes of ostensibly transcendental music is what you get, maybe, when you compress three hours to ten years of your life compressed, distilled, and crystalized into a repeatable musical event. Even that period of the creative process itself is a preliminary compression of the creator's life.
At an emotional level the reason the teleological argument can be so appealing is that we so readily see it in our own lives. We can see how much of our lives we put into investing in a new car or a newly formed relationship or a new child or a new career. Yes, I know where unbelievers will inevitably go with that but I trust you get that's not my essential point--the creative work is essentially a fractal, a microcosm that indicates the macrocosm of the person who made it.
What movies about the arts almost invariably get wrong is working on the assumption that it does not, in fact, actually take a lifetime of discipline and both external and internal exploration to produce a work of art. This is what makes the abrupt flashback in Ratatouille so poignant--Brad Bird, genius that he is, found a way to depict how a recipient of art was able to instantaneously grasp a metaphor from his experience that revealed not merely the finished product but the PROCESS. Ergo, Ratatouille is one of the very few films I've seen about the artist and the artistic process that gets most of the important things right. The film is luminous and beautiful without compromising the awkward and uncomfortable reality that the artist is generally working his or her ass off to arrive at what he or she would often consider mere competence, which, to us, is transcendant.
As an amateur composer and a guitarist I certainly notice how much of my life goes into working on what are surprisingly small pieces of music. I do not have a gift for the gigantic or the epic as those terms are generally understood. If I have any gift at all it is one acquired through decades of slow and tedious work that is nonetheless rewarding. If I were to attempt to describe what my work is like I would have to describe it as the studied epic miniature, a small work consisting of intertwined miniatures that reveal epic processes once you get past the light and fizzy surface of the individual parts.
If art, as Hitchcock reputedly said, is life with all the boring bits taken out of it then films about art and the artistic process are nearly impossible to make. This is because, ironically, to borrow an observation from a film, Russell in the film Up points out that it's all the boring things you do together that end up meaning the most and in the same way it is all the boring things about creating art that end up meaning the most in the actual creation of art, whatever that art may be. In other words whoever is faithful with little will be faithful with much and the popular narratives surrounding art tend make much of narratives of artist being, how do we put this, not very faithful.