Why would Apple want to write a completely new cross platform UI library, cloning the Mac OS desktop UI and give it away, or give away the Mac UI directly? How does that serve their interests?
The main purpose seems to be to enable developing back end server code in the same language and using the same service APIs as the client code on mobile and desktop.
I speculated when it first came out that internally they have a port for either Windows or Android or both, if for no other reason than to discover any architectural limitations that would preclude such a thing. But I also think it's very unlikely that SwiftUI will ever be released for any non-Apple platforms.
They already have a port of some of the core platform GUI elements to windows. It's used in the Windows version of iTunes. It's probably quite minimal though.
The main purpose seems to be to enable developing back end server code in the same language and using the same service APIs as the client code on mobile and desktop.