I would think (i.e. pure conjecture) that it's about economies of scale. Making enough smaller ships to make trains will costs more steel. Making a train that f

I would think (i.e. pure conjecture) that it's about economies of scale. Making enough smaller ships to make trains will costs more steel. Making a train that fills up a similar area in the ocean reduces the available storage per shipment. Follow this thought to "let's just make these trains as long as possible." You can imagine some wild consequences, both positive and negative.