It's the supply chain. Nearly all of the parts to make these electronics are created in the Asia-Pacific. In many cases that's all the way down to the raw materials being dug out of the ground. The fact is that these things are assembled there because it's immensely cost effective before labor even enters the equation.
Right now only 200 million or so finished iPhones make their way across the Pacific ocean each year. Switching to transporting each individual component, just so an American can assemble them, creates a huge increase in shipping costs and time. Imagine the separate shipments required for the hundreds of separate parts that make up a single iPhone. If you're making 100,000 at a time, that's 100,000 processors, 100,000 displays, 100,000 batteries, etc; now multiply that by 2000 to get to your 200 million total. Now put each of batches of components on a carrier ship and have it make it's way across the ocean. Each neatly wrapped separately to keep it safe until assembly. That is a giant amount of needless waste in shipping cost and time.
What if you want to be "green", as well as save money, and reuse all the packaging materials for your 2000 or so batches. Well put all that on a ship and send it all the way back to Asia.
All so Americans can have the pleasure of doing one of the most boring jobs available to humanity.