You know, the problem with writing a big story is that generally, only a tiny fraction of everything you brainstorm for your project actually ends up being in the finished product. At least, that’s how it’s supposed to work.
Perhaps, literally every idea you come up with is an immediate peak of brilliance, but usually any concept takes quite a few second thoughts and proper polishing. Think of it as an artist creating several variations of a character design to choose the best option.
As a result, it may take months and pages of notes to find the best solution for a subtle plotpoint that most of the readers won’t even notice, or the best way to phrase a dialog that consists of a couple lines.
So if something small like that takes so much time and effort to become properly fleshed out, you can imagine just how much work goes into the huge projects, with multiple storylines and a big cast of characters. And so instead of a polished small project you get a sloppy giant, as that kind of workload forces you to cut the corners somewhere. The more superheroes Marvel adds into their new Avenger movies, the less screentime and development each of them will get, unless the audience is ready to sit through a 4-hour screening.