No doubt in part because they benefit from the protection afforded by nation-states. Cruise ships tend to be filled with the citizens of powerful countries (or countries allied to powerful ones).

Mostly governments have done very little about piracy in Somalia.

Also at the scale you're talking about, small fast boats, a well armed private militia would be just as effective, as a billion dollar destroyer.

A few 3" guns will stop anything a pirate can throw at them.