Although I do agree that means there's less competition in the browser engine world.
The Mac front end uses macOS APIs and Apply is super selective which ones they port to Windows and WebKit would force them to open source this.
For it to make sense to open source Safari for Windows, they’d first have to pull out the AppKit bits and replace them with something else.
[1] https://appleinsider.com/articles/12/07/25/apple_kills_windo...