I know the original layout for Mount Rushmore was supposed to contain much more detail, like the bodies of the presidents, but had to be halted due to lack of funding. Why haven't we tried to finish the project since?
The artist is deceased. That's most of it- it's now considered a historical piece of art. If the Mona Lisa were intended to have another woman beside ML, we wouldn't go back and "finish" it for da Vinci.
Additionally the amount of carve-able granite left on the mountain is pretty sparse. It may be logistically impossible. Certainly not worth the risk.