It can do this only because Apple exercises such tight control over all platforms. When everything is developed under the same roof, it is significantly easier to make it, "Just work." On the other hand, making three, independently-developed platforms look the same is nearly impossible. On a windows or linux box, you cannot guarantee things like screen resolution, ppi, dimensions, specs, compatibility, and the innumerable other things that cause trouble.
There are almost no technical reasons, only political, for why we don't have a truly cross-platform, truly native UI framework by now.
The whole situation is bizarre and comical if you look at it from a distance:
Everybody ALREADY puts tremendous effort in making sure things look and work the same everywhere: Browsers!
Not just Apple, Google, Microsoft and Mozilla, but the thousands of third-party maintainers (like yes Electron) in this newly-created circle of Hel that is the JavaScript ecosystem.
And now we're trying to turn the browsers into operating systems!!
Everyone keeps trying to reinvent a wheel that doesn't travel very far to begin with (see all the missing features from web apps that native apps get, being discussed on this page.)
