They need a lot of customers to make it worthwhile to bring US costs down. Right now US production is mostly used for prototypes where high production costs don't matter as much as flexibility to make changes. If you need 10 made in the US that is good enough. If you need millions made in the US then it would be worth investing in all the things needed to manufacture something cheaply. With enough investment per-unit costs in the US can be lowest in the world - but few things are worth putting that much investment in when China already has most of what you need and the supply chain is shorter.

Everything I said about the US applies in some form to every other country to some extent.