Scale and throughput. Ships used to have cranes on deck and unload this way (look at pretty much any ocean liner, they had cranes on the forded for cargo). Containerisation improved throughput considerably, but added to latency. There is no way this is cheaper. Way more crew per tonne, the ship itself is almost certainly way more expensive for it capacity, and so on.

Not that I don’t think sailing is a bad way to move cargo, but I suspect if it does work, it won’t be small boats with crew.