Modern merchant vessels are already highly automated, to the extent that crew compensation is a fairly small component of total cargo costs. Trying to reduce the crew further would be chasing diminishing returns. While it's possible to build a vessel that navigates itself from port to port they will at least need some engineers on board to fix equipment when it breaks. It wouldn't be safe to leave a robot ship drifting powerless in mid-ocean. The crew is also able to do a lot of routine maintenance during transit. Without a crew, the ship would have to spend longer in port just to have enough maintenance time.