I see no reason for Apple to ever make the UI stuff available on other platforms, I can’t see how that would benefit them.

But there’s nothing stopping other UI libraries from making Swift bindings.

Tokamak[0] is really promising on this front

[0] https://github.com/TokamakUI/Tokamak

Apple surely must run a lot of linux machines internally, I imagine it would be nice for them to have internal services written in swift

That’s my understanding of why it’s on Linux (besides the community).

Apple uses tons of Linux servers, cloud and DC. Swift means they can write high performance networking stuff with memory safety and modern features in a language/library they already use internally instead of using yet another new language (or losing safety with C/C++/Obj-C).

Makes a lot of sense to me.

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.

As far as I know all of the UI stuff is dependant on iOS and macOS specific libraries. Apples UIKit and AppKit libraries are still proprietary here (and even have completely different capabilities depending on your platform and OS version).

It's not out of the question for someone to write a (e.g) GTK-backed SwiftUI API library.