I think the difference is that the actors in movies aren't trying to give the impression that they wrote the script, whereas musicians talk about "their" songs like they conceived them in a process of true artistic expression. Revealing that they are actually manufactured in a formulaic manner by the same people who wrote all the other hit songs can leave a sour taste in the mouth of a fan who is perhaps emotionally invested in a song or artist...
Perhaps in a few years, we'll read an article about how all of the tweets, interviews, and off-stage antics of pop stars are also the creative work of 4 middle aged guys from Scandinavia.
The impression that I form, on rare occasions when I'm exposed to what those musicians say about their work, is that it's just as formulaic as the work itself.
I agree that some fans might have a negative reaction to realizing that the performers don't write their own music, but it's hardly a secret. Songwriting credits exist for a reason.
Even if they do find that out, the music is still enjoyable. I'm literally listening to mass-produced music as I write this comment, fully aware that it's produced but still enjoying it.
Indeed, I think that the benefit of good songwriting outweighs the downside of knowing how songs are written. I'm a pretty good musician, and the one skill that I really envy is songwriting, which I just don't know how to do.
It'd have been a shame if Sinatra had tried to write his own songs, or if the Beach Boys had tried to record their own instrumental parts.
That sort of missed the point, though. The songwriting is a tiny part of a song. Most of the popularity value is in the audio and video production and in the persona of the person who mouths the words and stars in the video, and those aren't credited.
