The reason it is less expensive to manufacture in China is literal slavery. If iPhones were manufactured and assembled with no slave labour at all (mining, silicon forging, plastic manufacturing, plastic molding, metal casting, board manufacturing, assembly, and shipping) it would cost 2 or 3 times as much.
Breaking reliance on slave labour means moving away from disposable electronics and towards devices that last for 10+ years. But no one wants to use the same phone or computer for more than a couple years and even fewer people want to work in a manufacturing environment.