Comedic musical numbers fit well within comedy movies, but doing a drama with musical numbers is antithetical. The singing breaks the fourth wall too much, completely interrupting your suspension of disbelief. It’s impossible to take dramatic things seriously when they’re singing and dancing to/about it. You can break the fourth wall in comedies with music, because nobody watching is taking it seriously enough to think that the movie with the boner jokes can’t have a bit of musical parody.