Filmmaking is an insanely difficult process. There are so many people involved, each with their own creative vision, that it can be hard getting everyone on the same page. Plus, you've got to deal with issues ranging from financing to scheduling to on-set accidents.
With all the rewrites, test screenings, and last-minute edits, it's amazing that any film gets made at all. As a result, there are a lot of movies that are just mediocre. But every so often, everything lines up just perfectly. Get the right screenplay with the right director with the right cast and crew, and you just might get a perfect film. These movies are few and far between, but when they come along, you know right away you've found a flawless film.