There's a famous phenomenon in copyright known as the orphan works problem. It refers to the situation in which the copyright owner of a given work cannot be found. This effectively prevents others from using such a work as part of a new project. For example, if you want to make a movie based on a novel, you must first get permission from the novel's copyright holder. But if the novel is an orphan work, then you can't even find the copyright holder. Technically speaking, you could proceed without permission — but you would do so at your own risk. The copyright owner could emerge at any time and demand penalties. You might end up having to pay damages; worse, you might have to abandon or censor your derivative work, no matter how much effort you'd put into it.
Related to the problem of orphan works is another problem, much more serious, yet much less discussed. I call it the ghost works problem. Ghost works are all the works that never get made in the first place, or are made but not released, because copyright concerns prevent them either from being started or from being distributed. Every project that dares not base itself on an orphan work becomes a ghost work, but there are many more ghost works beyond that. Indeed, it would be fair to say that today most works are ghost works. That is, most works either don't exist or are not accessible, because copyright obstructs them. Whenever you walk into a bookstore, survey the shelves around you and imagine them to be 90% empty, for in a sense they are.