well of course we could - but i won't. if would like to write cross plattform stuff and not stuff that is only for one plattform available.
You could use some open-source crossplatform 3d engine, ogre3d for example, supports both d3d and opengl, when compiled under windows.
And there are many other advantages in using a 3rd party engine.
On the other hand migrating to a new engine is also a lot of work.
But I got UFOAI running, so d3d is not needed any longer