Just finished reading this
article, interview with Toy Story 3 writer Michael Arndt, long but fascinating piece at Thompson on Hollywood, so a real delight to learn of this other interview.
Re why are they such good storytellers, you raise such great points, Scott, can't add much, but my two initial responses are true for both design (what I know) and scriptwriting: they never lose sight of the heart of it (the story, and the audience, whom they hope to move) and they are ruthless with editing.
That Pixar makes such a wonderful work environment for their people is surely also a factor.