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.
* The ships are larger and more expensive and hence the add on expense is smaller, comparatively
* There's less traffic around and the traffic that is around is likely to have on board beacons, etc
* The ships already have more sophisticated instrumentation than trucks (radar, etc)