It's a sad fact that the majority of movie scripts will never be made. There are thousands of screenwriters slaving away at their desks morning, noon and night, but only a very small percentage will ever see their name in a credit list, let alone find regular work with a studio.
But that's not to say that only a small fraction of all a writers' output is actually worthy of being made. There are hundreds of excellent scripts that have sadly remained unmade for years, sometimes decades.
There are many reasons for their mistreatment, but it's usually because the script just isn't particularly marketable -- meaning the likelihood of the film making its money back is remote. Or, sometimes, a brilliant script is just too similar to an existing property. This happened with Manhatten Ghost Story, written by Ronald Bass (based on the novel by T.M Wright), which is a fantastic piece of work... that bears a strong similarity to M. Night Shyamalan's The Sixth Sense.
No plagiarism was involved on either side's part, it was just bad timing for Bass that he chose to adapat Wright's novel at the same time Shyamalan cooked up Sixth Sense. But hey, maybe in 15 years the shadow of The Sixth Sense won't quite be so overbearing and Bass' script will get a chance...
There's also a huge amount of novels and comic-books that should be adapted for the big-screen, including Alan Moore's graphic novel Watchmen, Arthur C. Clarke's Rendezvous With Rama and Fight Club author Chuck Palahniuk's follow-up Survivor. All of these are great reads and scream out to be made by visionary directors. A few famous names continue to circle these projects, but they never seem to get picked up...
If you have the time, take a look on the internet for some unproduced scripts. You might find you're reading the a blockbuster years before it even gets made! Simply Scripts is a good starting point...