In fairness, the chances of becoming a citizen is low. They wait over a decade just to get permanent residency, another 10-15 to get citizenship...so we’re not encouraging them to ‘become American’ and I’d expect that to trickle down to their kids.
If they have any change in employment, primary address, even if the business moves office...they have to go through a bunch of immigration hassles, more paperwork. Even the very good engineers I’ve met from there really have no plans for the long term.
Your story is not uncommon though, sorry to hear that. I saw the same at Visteon in Detroit. Once a flourishing engineering HQ, called ‘The Village’...now a ghost town, only a skeleton crew.