That's something that I have always wondered. And, having built a few of these myself on my free time for personal projects (starting with a university homework

That's something that I have always wondered. And, having built a few of these myself on my free time for personal projects (starting with a university homework), I have my $.02 It is really difficult to do this correctly. The simplest systems to build with are the ones where you have a large visualization window and you only need some knobs to control it. The things that you think are going to be tricky (say, skeuomorphic knobs) are actually almost trivial to implement, even if compared to standard GUI toolkits(but not browsers). Things that you have taken for granted (textboxes) require months to implement correctly. And you will still find corner cases, be it keystrokes(even without taking different operating systems into account). It is easy to make it look 'flashy'. It is surprisingly difficult to make it look 'good'. We see some beautiful UIs in games and we think our applications can look the same. They really can't, not without great graphic designers. And no matter how great it looks, it will look 'alien' compared to other applications in the system. If you need any kind of dynamic layout (which you normally do to handle resizing), you are now looking into an entire system and months of development. Native OS controls usually have ways to hook up to them from external apps. Not so when you are drawing yourself. So that breaks accessibility but also any chance of automation. All those drawbacks (and more I haven't touched) aside, there are benefits. They can be incredibly fast, faster than native OS controls. Rendering a few quads is no problem for any GPU. You can have incredible interactivity(this needs to be explored more by non-Games), since rendering is so fast, faster than the older days of blitting stuff in framebuffers(and a lot of it can be handled by the GPU). You can also implement transitions that are impossible to do in anything else (maybe except for web browsers). You can transition between 2D and 3D seamlessly. For color blindness you can just apply a very simple shader over the whole thing. I think there is a lot of potential hidden by this approach. Think of some the of GUIs imagined by works of science fiction. The main issue is that there's a lot of work to be done before you have anything functional and if you use something out of the box, it will be probably difficult to extend. That said, I've considered taking this idea to an absurd level and write, say, a text editor on something like Unreal Engine and see how far into absurdity we can extend this idea.