Note the word "only" which makes my question different question from this one.
If you see the Mr Bean TV series you will hear background laughs (a laugh track), but in Bean (the movie) there is no background laughter at all. Why the difference?
The usage of laugh tracks in a TV show dates back to early television and the necessity of shooting a scene multiple times. While the performances of cast and crew could be tailored to fit the scene, the "audience laugh" - the aural cue to the viewer that something funny had happened - couldn't be controlled as easily. Once audiences had seen that scene once or twice, the laughter would either be at the wrong time or too short, or worse no one would laugh at all since they'd seen the "punchline" multiple times.
The main reason behind the usage of laugh tracks is to replicate the experience of being with an audience while viewing. Watching TV at home is usually a solitary experience, so film-makers attempted to recreate the atmosphere of a theater by introducing laugh tracks. Eventually this expanded to include a range of emotive aural cues - applause lines, the "awww" moment, cheers and whistles during romantic scenes, etc.
More specifically, though, why are laugh tracks not used in film when they are so prevalent in TV? Largely because films are intended for the theater, and in the theater the viewer will be surrounded by an audience. In the theater, the rule is "laughter begets laughter", so if you can make a few people laugh or applaud, other audience members will pick up on that and produce genuine laughs as well. The same can be said of comedy shows or political rallies that use "plants" to generate the same effect - a few paid supporters who will laugh or cheer at the right time, thereby encouraging others to do the same. The same ethos applies to laugh tracks stimulating laughter in the "at home" audience.
The inclusion of laughing for a sitcom comes from the fact that many of them were originally filmed with a real in-studio audience just like a game or variety show. The show would include the laughter from the real audience.
Now shows that want to replicate that style will use quality, well produced laugh tracks rather than rely on a random result from a real audience. This allows directors to film several takes of scenes, and without a real audience, they can shoot out of order, etc. and still get the same, at-home laugh inducing laugh track. Sitcoms with a laugh track are still filmed in a way (on a single set) that makes it believable that a real audience is sill there.
Movies on the other hand were never filmed in front of a "live studio audience". Movies were and are always done on set or on location with multiple scenes and with post-shooting audio mixing. Movies don't have laugh tracks because it would be obviously fake and hokey.
In the 1950s and 1960s, most sitcoms were filmed in front of a live studio audience. Comedy performers prefer working with an audience so they will know what is funny and what is not. This was probably because radio comedies of the 1930s and 1940s were broadcast and recorded with live audiences. This helped performers ad-lib and pace the comedy.
By the 1970s, comedians like Woody Allen were already complaining that TV comedy laugh tracks were being overused.
TV sitcoms are usually rehearsed and performed like a play. They are filmed/taped in-sequence, so that the first scene is recorded first, the second scene is recorded second, and so on. This makes it easy for a live audience to participate.
Films are usually filmed out-of-order, based on the availability of locations and the actors' schedules. Until recently, films were mainly intended to be watched with an audience that would presumably be laughing. Comedians like Laurel & Hardy timed their routines so there were pauses after funny gags so that the audience's laughter woul not cover up ensuing dialog.
So basically, radio and TV shows traditionally had live audiences to provide laughter. Films never needed it. And it was easy to cheat and add laugh tracks to sitcoms that had no audience, or when the audience didn't laugh enough.