I know it used to be cross-platform, but at this point its clearly proprietary and is only available on macOS and iOS.

Apple uses their walled garden approach to limit a lot of things, including forcing all browsers on iOS to use WebKit to render. So it really limits other browsers ability to compete with Safari because they can't as easily add new features (including better access to ad-blocking). That's finally going to be changing, but mostly because of being forced to do so by the European Union. Apple is just fairly anti-competitive in certain ways is all, and certainly has been with their browser for a long time.