Musicals in Movies

First: Bing was fantastic in a fine understated manner in this film.

But... why is it, when Broadway musicals are shown in film they are turgid bores (this movie) or drop jaw examples of camp WTF? Valley of the Dolls, Staying Alive and The Fan are examples of this. I bet you readers can offer other examples.

I slept with you and you're in love with my husband. What the hell am I supposed to do with that?