https://www.inc.com/glenn-leibowitz/apple-ceo-tim-cook-this-is-number-1-reason-we-make-iphones-in-china-its-not-what-you-think.html
Quoting a speech he gave in China...
you find in China the intersection of craftsman kind of skill, and sophisticated robotics and the computer science world. That intersection, which is very rare to find anywhere, that kind of skill, is very important to our business...and the quantity of skill in one location and the type of skill it is.
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The products we do require really advanced tooling, and the precision that you have to have, the tooling and working with the materials that we do are state of the art. And the tooling skill is very deep here. In the U.S., you could have a meeting of tooling engineers and Im not sure we could fill the room. In China, you could fill multiple football fields.
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The vocational expertise is very very deep here, and I give the education system a lot of credit for continuing to push on that even when others were de-emphasizing vocational. Now I think many countries in the world have woke up and said this is a key thing and weve got to correct that. China called that right from the beginning.
So basically, the U.S. doesn't have what it needs to produce iPhones in quantity, no matter HOW much you're willing to pay to make it. The people and the skill sets aren't here. Now, maybe if we accepted a lot more immigrants...

, But also, from what I read elsewhere, even what we do have is scattered all over the country, whereas in China, as alluded to above ( "in one location" ). there are geographic areas where basically everything a company like Apple needs is located together. Huge numbers of people with the needed skills, and also, the related industries/factories nearby. I guess the kind of thing that comes with central planning.
Simply saying "we'll just pay more for labor, and then we can make it here" doesn't solve these problems.