It's because there are other sources of Covid. People can get exposed after the test, they can have Covid and not yet be positive, they can get Covid from shore excursions, etc.
But each valid positive test eliminates a possible Covid source.
So people can talk about missed cases, other diseases, cruises vs. Walmart, whatever. None of that gets around the fact that halting testing would mean more Covid sources aboard ships.
More Covid sources could mean more quarantining and disruption.
How much more? Wish we had those numbers.