Jack Nikas reported for The New York Times on why iPhones won’t be assembled in USA.

In China, Apple relied on factories that can produce vast quantities of custom screws on short notice. In Texas, where they say everything is bigger, it turned out the screw suppliers were not.

Tests of new versions of the computer were hamstrung because a 20-employee machine shop that Apple’s manufacturing contractor was relying on could produce at most 1,000 screws a day.

Remember the fidget spinner fad last year? How were the manufacturers able to produce so many of those in just such a short span of time? The product used bicycle parts and there are hundreds of factories in China producing bicycle components.

Stiff competition also means that suppliers would go the extra mile to provide better service and the shortest lead time to gain an edge over their competitors.