It's fun for classical music fans to recognize pieces from the repertoire featured in movies — and often, it's not all that difficult. You don't need to be a musicologist to recognize Orff's O Fortuna, or Barber's Adagio. Both are go-to favorites for filmmakers, who have used each in dozens of movies. A supercut video from Slate demonstrates the ubiquity of those two pieces, as well as Chopin's Nocturne in E Flat.
With innumerable classical recordings available, why don't filmmakers try a little harder to be original? Well, in some cases they do — the Slate video opens with Stanley Kubrick's famous use of Richard Strauss's Thus Spoke Zarathustra opening in 2001: A Space Odyssey. The 1896 tone poem wasn't familiar to a popular audience before the film's 1968 release, and is now so indelibly associated with the movie that no filmmaker would dare use it other than in reference to 2001.
Kubrick also used virtually unknown pieces by Ligeti and Khachaturian, helping to raise the profiles of those then-current composers. He additionally used Johann Strauss's Blue Danube waltz — a very familiar work, then as now, used to underline the graceful maneuvers of spacecraft.
That suggests one reason such nuggets keep cropping up in movies: oftentimes, filmmakers are specifically looking to evoke a sense of familiarity or nostalgia. (This is particularly true with films that feature their own original scores, and wouldn't need to use a pre-existing composition unless it was for deliberate effect.) Other times, as narrator Lydia Cornett notes in the Slate video, it's "simply lazy."
Cornett suggests some alternate selections for filmmakers who want that Orff or Barber feeling, but would prefer not to trod the same ground. Would Platoon be as good if Nielsen's Andante Lamentosa replaced Barber's Adagio? Watch the video and decide for yourself.