I asked that question once, apparently crews aren't armed and it's a big deal to start shooting at people at sea. There are videos of people fending off pirates (actual, sea based pirates) with water hoses.

I don't get that - it seems pretty obvious you could arm a crew and fend off some assholes with AK47s (you have the high ground, lots of corridors, time to prepare and can afford some .50's on the deck) - but then again, I'm so American I am practically a Bald Eagle.

escalation. Shipping companies don't want to have to worry about how much armament is enough. Also it blurs the lines between a cargo ship with defense against pirates to an armed ship that may have cargo. But yeah some ships now have mercenaries.