Why are the directors and not the screenwriters credited with Oscar prizes?

I don't really understand it. Recently Susanne Bier won an Oscar (best non English movie or something like that). But that kind of confuses me, cause isn't director and screenwriter like conductor and composer in music? I mean isn't the screenwriter the creator of the movie and the director more like the "guide"? So why is the director always credited? Please someone explain that to a filmnewbie.