Because some of the destinations still require it.  See my earlier post hypothesizing that RCI can do this for cruises 5 days and under because those itineraries don't visit the islands that still require a test.   This isn't something Carnival can do unilaterally across the board.  But they may be able to do it the same way RCI has done, if none of their current destinations for 5-day and under cruises require a pre-arrival test.