They build them there because all the supply chain is already there. In Shenzhen, the capacitors, resistors, PCBs and most chips are sourced in bulk from around the block. That's something no other country in the West or even East has and it's such a complex interconnected ecosystem that you can't replicate it anywhere else right now.
Most of the CNC, tooling and moulding craftsmen are there as well, the west has far too few left to be able to take over such volumes. Training so many can take years and that's assuming you even find enough people willing to be trained.