Sub contractors as you call them have been the norm for over a hundred years in Great Britain. If it has worked well for them I see no reason not to take advantage of the same here. The investment in tooling, education and time needed to master any task almost make it impossible for any person to do it all to a high level. Yes you named a half dozen who can but what you did not say that there are thousands of others that are average at best and too many are dreadful in truth.