[leot]

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).

[learc83]

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.