First of all, plenty of people actually did do just that. That is, several novels were written by various authors based on their own experiences in a role playing game.
Indeed, not only do we have a number of books and series of the vanilla fantasy type which credit their role-playing group and friends for the genesis of the story, we also have a slew of books which overtly involve people crossing from the "normal" world into their role-playing or fantasy world.
Everything depends on what someone means or wants when they talk about translating D&D to a movie. If you want a fantasy story, then the reference to tolkein and the recent six movies was incredibly appropriate. The entire fantasy genre owes a heavy debt to Tolkein, including all fantasy RPGs.
But to extend this thought, as others have stated, by now (decades later) there are TONS more sources to draw upon to create good Fantasy movies. Why try to blow the dust off a 30-year old script?
If instead you want a movie about teenagers playing D&D, you've got an entirely different challenge to create a compelling script, especially one that is believable, realistic and appeals to a broad audience.
How about a movie that spends the first hour with character generation? Or rage-quits? Or endless arguments about what WOULD be possible in the game-world? Or arguments about the difference between what the PLAYER knows vs. what the CHARACTER would know? (you know... where the GM argues the fireball is volume-based; so cast in cramped quarters would end up frying the party while the player argues the mage would have known that!)