First of all, what do you see around you that is not made in China? Probably you'll find some stuff made in Vietnam, Indonesia and other countries but most of everything is made in China. Mostly cost driven.

But as for miners there is another big reason, legal framework. You establish your company in China and your miners are late, they break, you don't honor your warranty, good luck for people trying to sue you. If you are based in the US and your miner comes out in a different shade of silver, you get your ass sued to hell.