Because an island isolation camp set up using the resources of a wealthy national government to deal with a highly contagious disease would somehow still not be able to duplicate the medical staff and facilities available aboard the cruise ship.
Of course! What possessed me to even suggest it?
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Yeah, an isolation camp in Army tents will just be paradise compared to the primitiveness of a cruise ship. The luxury of latrines, communal bathing facilities...all the comforts of home. No thanks!