As RealityChuck states, film festivals have a wealth of films that will otherwise never be seen by anyone else - usually for a very good reason.
I used to review films at the Berlin Film Fest, and for every gem in the rough I would see and praise, there were usually 20 bombs that deservedly never got any distribution. Some would have stars (from the local country) and there was obviously some money spent on production values, but the story sucked or the acting sucked or a combination of both.
One story I like to tell is the time I went to a film from India…screening room filled at the beginning, but as this film went on, the audience started to wander out of the theater. I am diligent and never walked out of a film, hoping there might be some great surprise/worthwhile moment at the end of the film. So, when the film ended, I was the ONLY person in the audience and the film had indeed stunk. As I walked out of the theater, there was the director, producer and a couple of the stars of the film who were all smiling and said, “Well, what did you think of the film?!”
Geez, didn’t they notice the other people fleeing in droves?
I was polite and complimented them on the sound, the lighting, a couple of set designs…pretty much every scrap of good I could dig up. People only hear what they want to hear and they all seemed to think I loved the film, despite not once mentioning the plot sucked and the acting was horrible and the film went on and on and on.
So yes, there are probably millions of films that were never screened except to other friends at film schools or the occasional film festival that is desperate for entries.
Sadly there are other smaller films that were actually quite good, but for various reasons (controversial subject matter, esoteric stories) were never picked up for distribution. It is really expensive to distribute a film, and unless you think you can make money with a wider audience, I can fully understand major distributors taking a pass. The good news is, those producers, directors and actors often get asked to be a part of some new project that might be a bigger, more commercial hit. So even though there are indeed many good, an mostly bad films, unreleased and sitting on some shelf somewhere, it is not necessarily a total waste of time and money for those involved. Ask any famous person in film and I am sure they can list a few movies they are not exactly proud of, nor wish to see distributed to the public today.
There was a great little film with Kiefer Sutherland and Reese Witherspoon called Freeway that cost $3 million to make and grossed under $300,000 in the theaters. I thought it was quirky and fun, but I doubt Keifer nor Reese rank this high on their film careers. This would be a great example of why many distributors might be gun shy at plunking down money for a film, even if it does have a cast of actors who are not unknown.