because it hasn't been profitable. The technology required, not only to produce but also to display such movies, has been too expensive to make economic sense to studios. But like all technologies, they become cheaper and better over time, and now these methods are becoming affordable to the big studios. The processing bang you get for your buck keeps doubling every 18 months, as per Moore's Law, so it's only a matter of time before CGI becomes a cheaper option than actors.
The film revolution will not just obviate many of those old filming jobs, it will create new jobs as a result of the increased capacity to make more films. A big studio that makes 50 movies per year today could make 500, generating more jobs than they initially killed. But these jobs will be of a different nature, as movies of the future will require more IT and design skills, and will largely be performed on computers. If I were a cameraman today, I'd get started retraining for a different workweek and learn the skills required tomorrow.
You can read more about this at the New York Times .