Because Shyamalan's stories are terrible. Someone else scripting it might save the dialogue (which has been poor since Signs--Signs sucked, but it's dialogue was workmanlike), but everything since Unbreakable has included major flaws that would be visible from something as bare-bones as a one-paragraph synopsis.*
*Maybe not The Village, which on paper might look okay. Signs, The Happening, and to a lesser extent Lady in the Water would all look back in summary, forgetting the bad execution.