Big cargo ships are already vulnerable to piracy and the handful of crew on board does little more than give the pirates some hostages.

If the ships were fully automated, and remote controlled, you could simply refuse to comply with the pirates and just keep sailing wherever you planned.

Under the assumption that the pirates could not compromise the command and control system, or simply cut the rudder and steer the ship manually.

But yes, it was these points that led me to suggest robot ships.

Why not robosubs?

Because sealing the thing would cost a lot more money. Containerised shipping is all about reducing overall expense.