Those are the prime libraries out there for 3D graphics, so you had better "go down" to OpenGL/DirectX if you want to do 3D.
Well, there's Ogre3d, Irrlicht, Panda3D, G3D... I could go on if you wanted me to.
And the fact is that they serve the same purpose as SDL - namely, they provide abstraction from OS and the user's choice between OpenGL/DirectX, and they are well-programmed and convenient. You wouldn't use OpenGL directly to implement a 2D game because you can do the same thing in SDL. Why use OpenGL (directly) for 3D when you can do the same thing in, for example, Ogre3D?

And I guess I ought to explain my choice of words - "go down" referred to going to a low-level library (in this case, OpenGL as compared to any of the aforementioned 3D libraries).