You have just posted exactly what I was wanting to post although you probably said it clearer than I would have.
I always have the same reaction when I see a quote attributed to an actor in a film rather than the writer of the film, too. Similarly comedians (like Bob Hope) are often attributed for the work of gag writers.
It's an interesting that this seldom happens in theatre, music or literature but often happens in film, comedy and speeches.
I wonder why this is? Is it simply that we assume live speakers to be speaking off-the-cuff and the same translates to film? If so then why don't we do the same for Theatre?
There's a story that a man approached Cary Grant on the street, and remarked "I wish I could be as smooth as you are on the screen". To which Cary replied "I wish I could be like that, too!"
Speechwriters are largely unknown, at least relative to the "performer" -- the same goes for comedy (let's say stand-up comedy since it's differentiated from theater and film in your list), and even film, quite often. On the converse, when you see a performance of "Hamlet", who's better-known? The high school kid bemoaning Horatio, or Shakespeare? I think it's just a convenient shortcut for people to attribute a quote to a performer rather than original author, in cases where the original author may be difficult to discern.
I mean, I can go to imdb.com and find out who wrote the screenplay for "Air Force One", but that still doesn't tell me for sure who originated the line "Get off my plane!" Harrison Ford may have ad-libbed it, or the director may have added it, for all we know.
Thus far I guess our society hasn't found it that necessary to create a culture of strict attribution.
Most popular music is not written by the person who performed it. Even prolific singer/songwriters have performed works that were written by other people that have gone on to become smash hits.
For theater, part of the reason may be that most theatrical productions are performed by numerous actors. Just think of how many Hamlets there have been over the years. Even modern theater is usually performed by numerous groups. Given this, it's pretty easy to say that the "definitive" work is the actual written play. However, when it comes to movies the "definitive" work is seen as the performance by the one actor that was in the movie.
Most popular music is not written by the person who performed it.
I think that varies by genre. It's certainly true around pop music. But I don't think so much for, say, metal.
On the other hand, having recently watched a few episodes of VH1's "Clasic Albums" series (at least I think that's what it was), I was really surprised to see the amount of creative input that came from the album's producers. I was surprised to see that the melody of Kiss's "Beth" was written by the producer, and that Bob Rock did so much with Metallica (unfortunately?) in building the songs for The Black Album, albeit it out of riffs written by the band.
It does vary, but I didn't want to pigeon hole pop. Mostly because it happens in a lot of other genres such as country/wester and even folk. It also occurs more in places such as Nashville, Austin and Memphis where there is a strong songwriting tradition.
"It's an interesting that this seldom happens in theatre, music or literature but often happens in film, comedy and speeches."
It happens all the time in music. Songwriters are seldom as well-known as performers, and you could argue that they do the lion's share of the "authorship" of a given song. Most of them arrange the music and write the lyrics. It's a rare instance where a songwriter becomes prominent enough, in his or her own right, to rise to the fame and fortune of his or her star performer. For every Quincy Jones or Leonard Bernstein, you have 100 songwriters and producers laboring in obscurity behind big-ticket artists. (In fairness, though, highly successful songwriters earn residuals on their work, and thus tend to be handsomely rewarded. They get the fortune, if not necessarily the fame or the popular attribution).
Theatre is an interesting exception to the general rule. Some of it is a function of time. We have every reason to believe, for instance, that William Shakespeare was less famous in his day than were the best-known actors who performed his work. But since there was no technology around back then to record the performances for posterity, the only things that survived were the written works -- attributed solely to the playwright.
The rise of credit for performer, rather than writer, probably has a lot to do with the rise of the recording and media industries.
In fairness, any truly great screenwriter or director will tell you that performers really do contribute enormously to the final product. A great screenplay is a great screenplay, but the finished film is a substantially different piece of work. A screenplay is to a film what an outline is to a paper, or what a wireframe is to a website. It's critical, but it's highly theoretical and open to myriad different interpretations. The interpretation that ended up on film, in this case, is the work of multiple "authors."
Theater, music, and literature tend to appeal to a "higher" class of people than film, comedy, and speaches. This higher class might care more and be more knowledgable about the subtelties of attribution.
No, I think you're creating a class distinction where it's completely unnecessary (and a weird thought to even have, I think).
Theater, music (I'll assume we're not talking about "popular" music, but something more like classical music here), and literature tend to have known authors, and tend to have a wide variety of performances. (Although literature is weird to include in this list, I think).
Film, comedy (since it's differentiated from theater and film here, let's say stand-up comedy) and speeches have known performers. Authors in film are sometimes relatively well-known, but I think attributions to actors rather than characters in films aren't that common.
I always have the same reaction when I see a quote attributed to an actor in a film rather than the writer of the film, too. Similarly comedians (like Bob Hope) are often attributed for the work of gag writers.
It's an interesting that this seldom happens in theatre, music or literature but often happens in film, comedy and speeches.
I wonder why this is? Is it simply that we assume live speakers to be speaking off-the-cuff and the same translates to film? If so then why don't we do the same for Theatre?