Regrettably, because it works. Humans are a social animals and when people around us are laughing, we laugh more easily. If you pick up on cues that something is funny, even if it's not, you are hardwired to be more likely to find it funny.
Exhibit A is Seinfeld. This is widely regarded as one of the funniest sitcoms of all time. But some people who disagreed put out videos of the pre-laugh track scenes, and when you watch them you realize...wow, Seinfeld is NOT funny. I watched the Soup Nazi scene, one of the most iconic Seinfeld scenes, without laugh track and it was...really unfunny.
The Laugh track is becoming less used in recent years (thank god), but it was an absolute STAPLE of TV for a long time.