Simple economics - if it was easy, everyone could do it.
I work on construction projects in China so I have an idea of the difficulties re: quality and expectations
I'd go so far as to say that it should be this way and will be this way. The more people who run headlong into the frustration and disappointment of this exercise, the sooner some talented people will realize that this is broken and fix it.
This means that, in order to afford to get these types of cost gain, you must produce a lot of something in order to justify it. Also, it means your engineers working with their's to optimize the design. This isn't as easy as it should be - like the interpreter throwing errors on line #X - because much of manufacturing engineering is still the art of try-and-see. Whoever is closer to the production line knows best.
I'd love to see a world where CAD files go in and fully made products are quoted, interactively optimized using simulations, and then produced on demand. However, these systems are hard to justify when large costs allow humans to do this for only slightly less efficiently than an amazingly complex computer system for this purpose.
We'll get there - look at the full-auto sites like Ponoko where you can get laser-cutting, 3D printing and CNC in this manner, but it's a while until full products can be made this way.