Responding to a summary of my WGA strike thoughts posted to Crimespace, mike D suggested writers should be paid more than actors because writers come up with the ideas and write the scripts while actors just perform them.
I'm not a screenwriter, but I'd guess the same way authors aren't fulfilled until their work is published, screenwriters aren't fulfilled until their work is performed by actors, and actors are most fulfilled when they get to play well-written characters.
The practical reality is, if a writer pulls out of a movie or a showrunner leaves a series, a new writer can step in and rewrite a script or retool a concept. If need be, the new writer can mimic the original writer's style closely enough to satisfy most viewers.
On the other hand, if an actor bows out of a movie or TV show, most of that actor's fans will probably follow suit, and whoever fills the actor's shoes has to build up a new fan base (e.g. George Clooney leaving White Jazz, Mandy Patinkin leaving Criminal Minds, James Caan leaving Las Vegas). The new actor cannot duplicate the original actor's presence or drawing power. This is why actors will always be paid more than writers.