Vaccinated doesn't mean virus free.
Vaccination just means you're less likely to have life threatening complications if you have COVID.
My roommate, vaccinated with Moderna in March, in August came down with COVID. A much milder case, but she still felt it's affects. Swollen glands, felt exhausted, sinus headache. Initially she thought it was allergies or sinus infection before telling me. Of course, then I needed to be tracked & tested too (super cautious at home; came out negative)
If the cruise companies were using just vaccination cards (which clearly can be forged) my roomie could have joined us all on board, spreading the virus.
The system isn't foolproof, but it's what we've got right now.