Because that's what it is.
When reading Infinite Jest I always wondered whether someone would ever make all the fake cultural references real. This thought occurred to me several times, when imaginary fiction or movies were mentioned in a book. But Wallace's universe in Infinite Jest is so immensely littered with non-existing movie plots it really screams for elaboration. Which is now being done, hurray!
In case you haven't read Infinite Jest and you are looking for a tip for your summer reading: please consider it. It is amazing.