The point of doing an operating system UI in a game engine is so that you can use that operating system UI within games written in that engine! There's several

The point of doing an operating system UI in a game engine is so that you can use that operating system UI within games written in that engine! There's several examples - Simulacra, Emily was Away, Hypnospace Outlaw - that use the interface of an (imaginary) computer as their primary framing device. Or it could be useful in an immersive-sim type game where there happen to be computers you can interact with; you could immediately see the applications in some kind of hacking-themed game.