I don't think it is the costs necessarily but the availability of people with the right skills. Even with the money, you can't just magic up qualified mariners.

Also, a significant amount of money is spent providing 24 hour cover of navigation in the bridge so I guess you could save something by automating a lot of it.

You are probably right that in the extreme, it would be better to have some humans to make sure your containers make it to their destination!