If you think about it, graphics cards specialize and do all the graphics work, while the CPU will do everything else. It would be extremely inefficient for the CPU to try and process the GPU's load, and incredibly difficult to make it work vice versa.
There are physX cards, which really don't help a whole lot, are rare to find now, and were expensive.
NVidia cards also support using a GPU as a physX card.
Not very useful IMO,