It may well be naive, but it's not as if the parts for the iPhone would take up much space on a plane. Aluminum, glass and battery are the largest components and I'm sure available widely in most countries.
As for labor, it's not a skilled job to place components.
I'd have thought that a lot of component parts are readily available in the area of China which specialises and is geared up to supply the industry. If you manufacture in the US and have to order in parts every time you need them, then you are adding time and increasing inefficiency as well as cost.
Presumably it isn't just the manual labour of putting them together - haven't people talked before about a lack of engineers etc with the required skill / knowledge in the US?
Again, it would maybe be a little naive to think electronics firms just need a bunch of unqualified people of the street to do manual assembly work.