That's probably because they categorize external labor not as labor but as assembly services. Parts, labor, and research and development are always expensive.Tim Cook has spoken in the past about the fact that the cost of labour is a small portion of the cost of their products. The issue is skills and volume of workers needed to do the work, these don't exist outside of Asia.
There are plenty of people in the U.S.A. who aren't working but could be. However, finding reliable people to do the job is another matter. There is a saying that is older than I am: "Don't buy a car that was built on a Monday or a Friday." Someone probably wasn't paying attention.