They started
reinventing the wheel, and the reason to use outside tool is because it will be cheaper, better and have support. Meanwhile they will have to maintain their mediocre solution themselves.
It doesn't matter how their UI framework work in game, in the end its just a dynamic texture rendered on some mesh and I am fairly sure i have way more "idea" than you will ever have, guess how? Well i worked with 3D engines and I am a dev myself
Been there done that, learned my lesson, there are several things you just don't reinvent, UI toolkits are one, microsoft tried multiple times, silverlight, dhtml, wpf, their own html engine, everything failed miserably. Company that is magnitudes bigger.
LOL at thinking VR is not interesting