A ton of UIs are built from the ground up. When you compare the logic and math involved to the logic and math used in 3D rendering or even 3D game logic, it's an absolute pushover, though you might feel like it's a huge hassle since in web development you get the UI logic, more or less, for free. You'll notice very quickly in game development that, until very recently and with the exception of graphics / sound libraries, you have to code everything from scratch.

It really depends on the library/engine you are using (if you're using one!) OGRE3D integrates well with several packages, Unity has a builtin GUI system that is stupendously inefficient, and the list goes on...