Very good point.
One thing that makes a laugh track sound realistic is something called a "titter track". Here's how it works: the titter track is a loop of individual people laugh quietly, it's always playing silently in the background, whenever a bigger laugh is put in after a joke, punchline, gag, whatever, the volume of the titter track is increased, allowing the bigger laugh to blend into it, then once the big laugh finishes, you ease off the titter track, thus making it sound like the audience is settling down after their big laugh, as opposed to a fade-in and fade-out.
I didn't learn that until the last year or so, but I began to utilize that with my recent videos within the last year. A friend of mine, who's also a laugh track nerd, managed to actually completely replicate, titter-by-titter, the titter track that was in used for all sitcoms during the 1969-1970 television season, and sent it my way for use. Since then, we've managed to also replicate titter tracks from 1967-68, 1970-72, 72-74, 74-76.