Because Hugh Jackman and Sutton Foster are great performers that people are eager to see in anything, and the vehicle doesn't really matter that much?
Because people are capable of differentiating between fact and fiction, fact and fantasy, and can laugh at something that happens in fiction that they would not laugh at in real life? I promise you, even in 1957 everybody knew that The Music Man is a fantasy deep-fried as a carnival midway corn dog in nostalgia and wish-fulfillment. Most musicals are (with or without the nostalgia part). And very little that happens in most musical or non-musical comedies is something that one might want to happen in real life--that's what makes them enjoyably funny, that they make us just uncomfortable enough to laugh.
Because wise and experienced people might--like you and me both--like to see human nature change for the better overnight, but know that it isn't going to (because evolution takes time), and therefore prefer to spend the intervening time laughing about human foibles rather than scolding about them?