I know a bit about this having had things assembled in China for my company in the past. The reason they don't use automation for this kind of assembly (Smart Phones) is because the tolerances are too small. The phones require 0.01mm tolerance on almost every part.
The machines used in car manufacturing only have a tolerance of 0.2mm which is fine for cars as many parts will shift more than that just when being driven. But for the phones that's just too much, chips will not seat properly, screens won't close, seams won't be seamless.
This is why all the phones and consoles and most electronics are assembled by humans. They use automated chip placers when constructing the logic boards but when it comes time to put those in the cases and make all the button casts hit their switches and so on you need a human to make sure it's all going together properly. You need a human to be able to run their finger along a seam and feel an unevenness.
As to why Apple use China instead of USA for this is one of manpower. Apple usually employs 750,000 chinese assembly workers. But when the iPhone 6 and 6 Plus launched and they were unable to meet with demand they instantly (in 4 days) boosted that number to 1.05 Million. For them using China isn't about the price of labour (that's just a nice bonus) the reason is the swarm ability of Foxconn, Pegatron and their other partners to hire hundreds of thousands of workers at a moments notice.
Now I've spoken about the iPhone in the above examples, this is a product which is produced by both Pegatron and Foxconn in China and Foxconn in Brazil for the south american market (Brazil has extremely high import taxes which is why they have a Foxconn factory there now). But if we look at the Mac Pro and iMac which are desktop computers these are actually not manufactured solely in China. The Mac Pro in-fact is only manufactured in the USA.
The reason for this is these are not high volume products. They sell probably 50,000 Mac Pro's a year and maybe 300,000 iMacs. The iPhone by comparison will sell 70 Million units in just this quarter alone.
I actually don't think Apple could manufacture the iPhone in the USA and get the same volume (1.25 Million phones coming off the assembly line per day) as they can in China.
I was thinking about this today and I think actually that Apple should stay in China but they should clean up their act. Their employees should see massive pay rises, the highest in China for an assembly job. $1 an hour is a joke when you consider iPhones retail for $1,000+ and I think they should employ more people on the factory floors to make sure everything is being run ethically which right now it is clearly not.
One last thing I wanted to mention is this graph from Apples website:
This graph shows the compliance of Apples manufacturing partners (Foxconn and Pegatron) in Apples 60 hour work week stipulation. You can see in the graph that it's almost 100% until it hits September. The release of the iPhone 6 and 6 Plus. Then it falls dramatically as these companies push their workers to work way beyond agreed upon hours to manufacture more iPhones. Apple during September as these numbers came in should have been penalising and publicly condemning what they saw but instead? silence. That is unacceptable.
For all the profit Apple is raking in I really don't feel they are doing enough for the people that make these products. It is appalling. You know the part that really disgusts me about this is the suicide nets. They literally have employees throwing themselves off buildings because of their working conditions and Foxconn's solution is to put up nets to catch people who jump. Terrible.