High volume low margin production goes to low cost countries. Equipment and skills available in US transition to higher margin value-added production. Even if manufacturers want to move production back to the US, investment in automation and appropriate talent is needed – and that’s not necessarily quick to pull off.
Read more about the prospects of Apple manufacturing in the US in an article from the NY Times.
Image credit: https://static01.nyt.com/images/2019/02/02/business/02apple05/29apple05-articleLarge.jpg