Why do American movies so often show people cutting the crust off sandwiches?

As a non-American but someone who grew up with a mix of cultures (my own mixed background and American), I've always wondered—why do Americans in movies/TV shows sometimes cut the crust off their sandwiches? I haven't seen this particular quirk in any other cinema, so I'm assuming it's a purely American thing. So my question is—why? Can anyone explain?