Why doesn't an orchestra just start jamming, with no score or sheet music? It's just notes on paper.
Why don't construction workers show up on a jobsite and start hammering without waiting for plans? It's just lines on paper.
Why don't...you get the idea.
Why don't screenwriters just film their own movies? Why don't composers just form their own orchestras? Why don't architects just build their own skyscrapers?
The screenwriter example should make it obvious why these song-writers' complaints are baseless. A movie filmed by amateur videographers with no editing experience, no acting talent, and no marketing is not going to achieve mainstream success no matter how good the writing is. Meanwhile movies with terrible writing rake in huge numbers at the box office just because they have big name actors and top-of-the-line production quality (ex: the Avengers). This is because the script isn't particularly important to the success of the movie. Hence why the script writer doesn't get paid a large percentage of royalties.
If the quality of song-writing was a key determinant in the success of a song, the song-writers would have leverage to negotiate better royalties. But it isn't, so they don't. If you don't like it, stop listening to pop music. There are plenty of struggling artists playing their own compositions in dive bars on friday nights that would love your support.
You sure it's just that? All the DC superhero movies also have big name actors, top-of-the-line production quality, heavy marketing and merchandise tie-ins. Yet those movies have been largely duds. They don't have the same resonance in contemporary popular culture as the Marvel movies. Or ratings by moviegoers and film critics. They're generally considered dour, stodgy, weirdly paced, illogical, or lacking soul.
The scripts for the MCU movies won't win any film awards with the "ivory tower film snob"-type of moviegoer. But they're outstanding examples of the craft of screenwriting for their genre. And I'm speaking as someone who generally finds superhero movies and comic books, including the Marvel ones, kinda tedious. I don't listen to pop music either.
The MCU movies have wit, humor, sentiment, perfect pacing, and a tone appropriate to the characters (Guardians of the Galaxy movies are silly and playful, Captain America movies are more serious, Thor movies have an epic/Shakespearean feel). They do a good job of staying faithful to the source material while still being comprehensible to casual watchers. You think elves do all that? It takes serious writing skill, and every screenwriter isn't capable of it.
> A movie filmed by amateur videographers with no editing experience, no acting talent, and no marketing is not going to achieve mainstream success no matter how good the writing is.
"Professional movie-making requires multiple specialists" is not breaking news to anyone. The topic of discussion here is one of the specialists don't want to share credit with people who weren't involved in their work. Would it be OK if an actor demanded a makeup artist credit because they ran a comb through their own hair on set once?