The refit would probably cost almost as much as a new ship. Adding elevators for the planes, support for the air wing (mechanics shops, electronics shops, paint and fab shops), air operations equipment and support, ordnance support for the air wing, defensive setups for the ship (guns, missile, rockets) as well as the refit to build a launch/recovery setup on deck, much larger crew setups to handle everything.
On my carrier, the USS Eisenhower, one of the first super carriers, we had 5000 people on board, 7000 when the airwing came on board.
With all that, you still end up with a very slow carrier that has been "Mickey Moused" together to do a job. Whats a few million more for a new ship?