Why not? What restricts a US citizen to be based outside the US? Atlas had a base in London Stansted, I think. CX has bases in the US.
You are correct, UAL had a London crew base, PanAm Germany, HA in Athens Greece, I think FedEx in the Phiippines.
Very easy to find pilots at a U.S. carrier that would jump at the chance to live overseas for awhile.