I realized after writing my post that I had not expressed my thought completely. Perhaps instead of calling it “rules of the genre” I should have called it “rules of expression.” Different art forms have different rules about how one’s thoughts and feelings are expressed. For example, in poetry there are rules about rhyme and meter. In ballet, there are rules about movement and music without words. In silent films, the rules of expression involve images, mime, and music. I think musicals simply create a different world, with different rules. Just like a fantasy creates a different world with its own rules: magic and dragons and special abilities. I guess I feel that if one can accept magic, why not song?
Though I totally understand if one is not into dance or song. If you don’t like ballet, then The Red Shoes is probably not for you. If you don’t like lots of action/fight sequences, then probably not Avengers. Do you not like gangster violence? Probably not Scarface. But that puts it down to preferences, not something inherently wrong with musicals.