In short, because they have to think about every little thing in a movie. As one said himself, 'Every little thing has symbolism in a movie, so you have to write-in every-little-thing.'
And it's interesting to hear about every little thing, just in case (being a dork who hasn't yet 'made it big') you missed the symbolism put in some little-thing, or you forgot how one big symbol (like the entrance of a key character, etc.) ties into a prequel-manifestation of that symbol.
In both Dead Man's Chest and Curse of the Black Pearl, the writers' DVD-commentary told you how the movies referenced past pirate-movies and/or -history. I noticed in Chest (and maybe in Pearl a little) the writers also pointed out the OBVIOUS things ... 'Ooooh, this is where Elizabeth betrays Jack.' ... 'Ooooh, Will tried to make a deal with the devil here.' ... ... ... And I'm thinking, 'Did you see "Romeo and Juliet" and kinda suppose that the two kids might`a liked each other a little? Did you have to inform all of your friends that that was what your guess was?'
Dude, we're watching the 'writers'-commentary' here to hear the stuff we didn't figure out on our own; main plotline stuff, if we didn't perceive that before, was badly written!! Believe in yourselves a little more, you dorks!
As for the movie itself, it was beautiful! Not only was it the amalgamation of all the contributing imaginations involved, but--unlike Lord of the Rings--it was MORE than a collaborative interpretation of one man's writings! It drew upon the knowledge (both creative- and historical-) of each contributor for its pure-source (rather than just for interpretation).