American manufacturing left for a very simple reason. Your annual health insurance premiums, just your premiums is the same as a Vietnamese factory worker's entire salary for a year. The average American sets on fire about 150% of the annual pay of that same Vietnamese factory worker's on direct car ownership expenses (insurance, inarguably artificially cheap US gasoline, licensing, parking, maintenance). Even a single person's apartment rent is 200% of that Vietnamese factory worker's annual wages. I can keep going regarding 401K matching and student loans and so on.
Is an American more or less productive than an overseas worker? Maybe. But "maybe" is only a little bit....not 5-10X more productive. Americans engineered an extremely expensive society to live where government doesn't socialize the social-service costs via taxes; it instead was "privatized"--which IRL means it falls on employers to subsidize employee's private for-profit market purchase of those same goods/services via inflated prices which eats into profit margin.
Or... 5000 IQ idea...they just don't play that game and offshore everything they can.
And the heck of it is...Americans who did this to ourselves (or ancestors, you get the drift), blame corporations for doing the economically rational (albeit for us citizens undesirable) thing; and foreign states for not self-inflicting this kind of neoliberal policy on themselves. The USA doesn't blame their own self-inflicted policies whose effects they loudly complain about (cost of college loans, cost of cars, cost of medical care, and so on)
