I think a lot of people don't want to miss out on their vacation because they get sick, and are afraid they'd have to cancel and lose everything. Thus, they go even when sick.
One of the best ways NCL can prevent COVID cases aboard the ship is to continue to retain the policy where if you test positive within 14 days of your cruise, you get a 100% FCC.
(yes, I know, "You should get insurance"...everyone should, but I'd say most people don't...combine that with a fairly rigid cancellation policy, and you'll get sick people boarding cruise ships).