Most of the Linux machines inside Apple are tucked away in data centers, they're not sitting on engineers' desks (there's always exceptions). The people writing

Most of the Linux machines inside Apple are tucked away in data centers, they're not sitting on engineers' desks (there's always exceptions). The people writing stuff that will run on Linux are usually writing in languages that are already cross platform. The stuff running on Linux inside Apple has a pretty good chance of being Java or Python (or Ruby for some teams). These languages are usually chosen for their ecosystem. There's not much of a Swift ecosystem except the proprietary one on macOS and iOS. Without all the frameworks available on macOS there's not a huge Swift ecosystem to leverage.