Could it be that the writers regard them as auteurs bringing their visions to the silver screens? Or are they a little intimidated by the leaders on the set? Maybe it is simply because the adapted and original screenplay categories are the places to honor them when they have no shot at claiming a Best Director or Best Picture trophy.
And perhaps it all just comes down to simple math. The writers branch has 377 members making the magic number of #1 votes just 63. While one can belong to only branch of the academy, anyone who has contended for a writing award is eligible to take part in the nominations process. That means a whole slew of directors can cast ballots for the writing nominees. (The academy as a whole votes for the winners.)
Last year, both writing categories were claimed by their film's helmers: Woody Allen won Best Original Screenplay with "Midnight in Paris" while Alexander Payne (along with co-writers Nat Faxon and Jim Rash) prevailed in Best Adapted Screenplay for "The Descendants."
In the past decade, almost half of the writing victories (nine of 20) were by directors who either wrote their scripts solo or with co-writers. In addition to Allen and Payne, there were: Pedro Almodovar ("Talk to Her," 2002), Sofia Coppola ("Lost in Translation," 2003), Peter Jackson ("The Lord of the Rings: The Return of the King," 2003), Michel Gondry ("Eternal Sunshine of the Spotless Mind," 2004), Payne ("Sideways," 2004), Paul Haggis ("Crash," 2005), and Joel and Ethan Coen ("No Country for Old Men," 2007).