You want to handle the aircraft as it was initially designed. This means that even though you could have proper CG location with internal ballast, you might fall out of design elsewhere, like in mass distribution. For example, consider the location of the engines vs the landing gear. There definitely were a lot of design considerations in choosing how to support the aircraft on the landing gear. It appears the gear are inboard of the engines (or where they normally would be) in the image shown. By instead moving all the weight inside the plane, you would be creating a different bending stress distribution, as all the engine mass, originally outboard of the gear, is now very much so inboard.
As a dramatic (more fun) example, imagine a scenario in which the ground crew, having fun with the plane, somehow managed to load the ballast in the correct CG location, but all the way out on the wing tip. I hope you could see that the wing wasn't originally designed with the necessary structure to support that weight all the way at the tip. The tips would droop down to the floor (if the wing didn't totally fail), and then scrape all the way to the next hangar area... :) Imagine then tossing the keys of your bent or drooping A330 to your dumbfounded buyer. "She's all yours".