Game engines are designed around game loops executing code every frame and not around power efficient layout caching.

Orthrographic hierarchies are how UIs are usually laid out and its a mild pain to move to a system that's depth sorted, with custom shaders etc. You can do more but you need to think about more and its harder for the UI system to know what parts of the screen can be redrawn. Games usually just draw it all every frame.

Specifically for a game engines, they usually don't do things like OS integration for accessibility, subpixel font rendering, that sort of thing. In theory they could but usually game engines seem to roll their own system for this.