The problem is that on the sheer scale of Apple's worldwide repair needs (if every big repair was sent to a US location), they'd need to have tens of thousands

The problem is that on the sheer scale of Apple's worldwide repair needs (if every big repair was sent to a US location), they'd need to have tens of thousands of highly-paid technicians who are good at soldering, and make very few mistakes (also, add the cost of new machines for every failed repair). Apple's already struggling with "lego-block repairs" (take out bad motherboard, put new one in), there's no way they could do this at scale. (See https://www.reddit.com/r/apple/comments/acqwut/do_you_know_what_happens_to_your_macbook_when_you/ https://www.reddit.com/r/apple/comments/acqwut/do_you_know_w... and more recent reporting on CSAT Solutions's incompetence, poor incentives, and very high turnover).