I did read the whole thread, and I picked on your because it's the most unrepentant of the outsourcers.
There is not even a lack of engineers nor a lack of idle factory space, nor a lack of robot technology to build them. There is a lack of desire to on-shore the the revenue.
Actually, I think there is a lack of capital (money). Any company bidding on supplying Apple with it's parts would need to show a fully functional assembly line, with the ability to move massive amounts of parts into the factory and then the finished product out. And they would have to show Apple that they have the ability to retool the plant quickly and efficiently to deal with last minute changes.
They also would need to demonstrate that they have recruited, trained, and are functionally staffed.
Any company is free to do this, and to bid on the contracts. Who - in their right mind - would invest hundreds of $$millions in acquiring the land (near a major port where land values are high), building the factory (or revamping an existing building), building the assembly lines, etc etc with absolutely no guarantees that they will ever win a contract.
Just the time it takes to move the parts from their factories in Asia by ship to a US assembly line is a disincentive. That is inventory (money) tied up while the part is in transit. One of the advantages that Foxconn has is that a large proportion of the parts inventory (money) spends a couple of hours to a day or two on a truck. The amount of money they have tied up in inventory is minimal.
One of the things that Tim Cook is renowned at is keeping the money Apple has tied up in inventory to a minimum. I think if an iWidget spends more than a week from assembly to being sold, Tim is unhappy. Any plant in North America means parts are now in-transit for days or weeks instead of hours days.
Also keep in mind that a North American assembly plant might be placed in Mexico or Canada, under NAFTA. Mexico has cheaper labour and cheaper land. Just building the factory and acquiring the land would be astronomically cheaper in Mexico. Canada has Vancouver, the closest major port to Asia - linked to the US with highways and rail-lines that have been upgraded specifically to move goods from ships arriving from Asia to US markets. Vancouver is about a day closer to Asia than even Seattle. Canada also has the skilled work force for a highly automated factory. What Vancouver doesn't have is cheap land. But Prince Rupert does, that port is even closer to Asia than Vancouver - and it has the rail-lines. Plus cheap power.
I doubt anyone is going to build an assembly line in Canada, or Mexico, or the US for that matter. The upfront cost is huge. Foxconn can do it because they are simply expanding on their existing factory network.
I only brought Canada and Mexico into this post simply to show that there is no 'divine right' that dictates Americans must work. They have to compete for the jobs like everyone else, and they may lose out on those jobs when someone has a better proposal.
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With all of the above said.... The one company that might decide to build the infrastructure domestically in order to bid on the job - is Apple itself. At the moment, it makes sense for Apple to build these things in China - strictly from a financial point of view. However, financially, Apple is vulnerable (very vulnerable!) to any kind of dispute between the US and China.
It appears that at the moment Foxconn and Apple believe they can influence their respective governments enough to keep frictions to a minimal level. The US is also bound by agreements to Taiwan and to Japan, both of whom have their own issues with mainland China. I could see Apple deciding to move some core assembly operations out of China so that if the supply lines are cut, Apple would have some assembly operations that they could expand and build on in order to stay in business. It would still be hugely disruptive to Apple to do this, but at least they would have something to build up from instead of having to start from scratch.
I know that Foxconn has a plant in Brazil now. Do they have one in Mexico? I seem to recall reading about one there, but could be wrong. I could see them trying out an automated plant in Germany as well.
Just speculating...