Because Apple uses those people for cheap labor..they would NEVER have an Apple plant here in the US, would cost them too much so instead they want to hire cheap labor while they get stinkin rich
Apple already has Apple manufacturing plants in the US in Elk Grove, California, building iMacs, and in Austin, Texas, building Mac Pros. These have been here for years. Apple is also promoting Foxconn building a $2 billion plant in Pennsylvania to manufacture iPhones. This has been in the works since June of 2016. Sorry to burst your bubble.
The sad part is an iPhone made in the USA would not cost much more then an Asian made one. It would be hard to notice the difference in price for a $600.00+ phone.