Why can't you do an opic nerve transplant for blindness?

Currently impossible: The optic nerve has over a million precisely aligned fibers. No technique can cause a single nerve to reconnect. To transplant an optic nerve would require these over a million fibers each to reconnect and in the precise manner. This is the same problem as the unfortunate folks who have severed their spinal column and are paralyzed. Much work needs to be done in this arena.

Good question: Neural transplants such as optic nerves would involve thousands of axons to line up precisely. Not possible as yet. Maybe in the future.