I've worked on ships that big and helped finish out one in the shipyard (admittedly, computer systems). Everything heavy is at the bottom of the ship. Engines, oil, water, it's all under the water line. The lower half of the ship is made of steel while the higher decks are aluminum. I've seen some of those ships go to 45 degrees with no problem.
Also, very few cruiseships spend time in the open ocean. Mainly Atlantic and Pacific crossings, which are minimized to as few as possible. Contrary to the popular image the cruise industry is not in the business of transoceanic travel. Ninety-five percent of the time these ships are in the relatively calmer settings of the Caribbean and Mediterranean seas providing vacations. You'll notice that even Alaskan cruises tend to stay in the inner passages as much as feasible. Cruiseships steer well around storms. Heck, once a ship I was on delayed entering Tampa bay by 6 hours due to fog.
Most of the accidents (not including the obvious individuals going overboard for various reasons) involving cruise ships come down to poor maintenance/training and ego, as demonstrated by the Cost Concordia's captain. The ships themselves are marvels of engineering and much safer than you think.
As stated by other posters, freak occurrences like rogue waves can only mitigated to a point. Look how well Japan was prepared for earthquakes/tsunami and yet they still had a major catastrophe. It's just part of living on planet Earth.