Their volumes are high enough that they will literally build an entire factory from scratch to produce a single product line, they are far enough up the luxury ladder that a few extra dollar in labour won't hurt them too badly, and the contracts with their suppliers are significant enough that they don't need the short supply lines of a Shenzhen and can just demand their suppliers Get It Done.

Having a domestic factory won't hurt Apple, and with an erratic President who'll flip on tariffs twice a week it's a sensible hedge against his inevitable next meltdown.