To be fair, I'm not sure how cruises work. But I would assume that their housekeeping has additional staff at their primary port of call and/or, more likely it's outsourced at the primary port of call. Meaning they staff for the routine mid-stay cleanings and don't get the staff required for a full turnaround that is much more labor intensive than a mid-stay clean at a hotel.
If they did turnaround every 2 days, and staffed for that- Let's pretend that would require -say 10 for easy math- people for that turnaround. And during a mid-night stay of simple picking up and making bed it would take 5 people for all the rooms, so the other 5 are twiddling their thumbs (or all 10 are just half working). Or, they just have a staff of 7-8 and can stay optimally staffed.