Modern car doors are usually sheet metal with a large amount of forming to give a shape (close to) what the designer wanted. It's difficult, large, expensive, and the knowledge required already exists in outside suppliers. Maybe someday they'll do it in house.

I recommend the book "Car"[1] about the redesign of the Ford Taurus twenty years ago. One of the topics is how hard it was to actually manufacture the (at the time) very complex curves of the exterior.

[1] http://www.amazon.com/Car-American-Workplace-Mary-Walton/dp/...