I suspect that a large part of the issue is just the procurement process here in the US. In China, Foxconn or whoever is a big huge company that Apple can deal with on equal terms and Foxconn handles the purchasing of these parts from thousands of suppliers from large to one man band.
But, it sounds like here in the US, they contracted with a much smaller company to do the assembly work. That company in turn likely has some crazy hoops that prospective suppliers are jumping though.
It seems that in China, you can be a tiny, possibly one man, company and sell right to the mother ship. But here in the US, the food chain just doesn't work that way. I guarantee that there is no way a company like Apple is buying anything directly from a 20 man shop. No way.
There may well be a hundred 20 man shops in Texas that can make 28,000 screws. But, the ability to make the parts is small part of the puzzle. I'm sure they want ISO certs, material certs, a quality plan, an onsite audit or 10, sample parts, ownership of the tooling, NDAs out the ass, fixed pricing agreements, massive liability insurance, penalties for late or defective parts, and 100 days to pay.
So, how many shops want to go through all the pain an suffering to fill what is probably a <$2000 order?
I used to work for a 150 man foundry. We made parts for Cat, Allison Transmissions, Cummins, and some other huge evil corporations. Even with sales in the millions, it was often not worth it due to the massive amount of bullshit we had to deal with. We had several full time employees who did nothing but pointless paperwork for these big outfits.
These big companies survive in spite of themselves.