From an "interfacing with the OS" point of view, a game engine interacts differently. Most desktop apps ask the OS "please render this text here, with this font" and the OS takes care of that. A game engine just asks the OS "here is a canvas I have drawn, please just put these pixels on the screen". The OS doesn't know that a chunk of pixels is text, so you can't select it, copy and paste it, or screen reader it by default.

This is the same difference between an system like Flutter and native App toolkits (I'm going to talk about Flutter because I know more about it). It has massive advantages on portability - Flutter rendering is very predictable on different phones because the only thing that can vary is screen size. The subtle variations between OS versions and phone manufacturer UI skins don't screw things up.

There are downsides: You need to implement all the common stuff that you'd normally get for free. This includes native look and feel - Flutter has to have all the Android and Apple widgets. It also includes text rendering accessibility, etc. etc.