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Probably because it costs money to move ships, crew etc. If a company moved a ship from Alaska to the Med someone would complain because the ship wasn't in Alaska - human nature.
They do move ships about it is called a repostioning cruise you can board a ship in Fort Lauderdale and it might call at a couple of places before crossing the Atlantic calling in at Tenerife and a few other ports before arriving at its new crusing area in Europe these trip can proof to be good value for money if you ever fancied such a cruise usually the move from the Caribbean to Europe is April/May and then going back again around October time. There are other movements as will which could include going to Alaska or heading for Australia etc. This way the cost of moving is paid by those who opt for such a cruise.
Yes RCIs Splendour & Brilliance OS do seem to have been in the Med now for a number of years.