Many of the classics were based on a cartesian point and grid system, some of them even had grid devices that they used to  guide them in their work--3D is essentially based on a cartesian as well.  It is manipulating objects in 3D space.  Art is evolving.  People doing strictly digital paintings, Corel Painter IX essentially a painting program and Photoshop even has been used to create completely digital paintings that emulate real world painting.

I think it depends on what the goal of a 3D artist is as to whether it's photorealism or a more traditional art media work.  Photorealism even employs post work and compositing.

If you think that people aren't using 3D tools to create art equivalent to Boris and others (you forgot Frazetta BTW), think again.  Pick up Digital Fantasy Art, Digital Horror Art, and Digital Sci Fi art--many of the works in those books are as good as if not better in some cases as pictures created with digital media.  The average reader who sees these illustrations would find it impossible to say they handn't been created with conventional media.   In fact there was a reproduction on this web site recently of  a Maxfield Parish picture that was almost indistinguishable from a real one.