Because China has 1.44 billion consumers and the USA only has 330 million, and companies like Apple in the long run can sell more iPhones in China because there are more people to buy them. The Chinese see Western brands as more desirable and authentic, much like many affluent Americans would prefer a foreign Mercedes to a domestic Cadillac.
Starting pay for Chinese workers assembling iPhones is $3.15 per hour[1].
China does not have as strong of environmental regulations so massive toxic lakes can be produced there extracting rare earths like neodymium[2].
Chinese women have more children than American women because in America we are constantly told the earth is overpopulated and man is destroying the climate, and in America we are told it is medieval for women to focus on being mothers who produce and raise children who ultimately become the workers and consumers that are a nation's true wealth. (The goose that lays golden eggs is more valuable than the golden eggs.) But if a woman enters the workforce today instead of having children her immediate production and consumption can improve this quarter's profits, whereas if she instead has 4 children who won't enter the workforce until two decades in the future, well, that's two decades in the future and here in the West investors won't wait two decades for returns.
So we must increasingly have non-American women give birth to the workers who make and consume Apple products and other Western brands.
[2] https://www.bbc.com/future/article/20150402-the-worst-place-...