Cargo ships as currently built cede the entire middle section of the boat to cranes. Cranes they won't see until the end of their voyage, or the beginning of the next. The whole rest of the time you can't use the vertical axis for anything else.

Things that slide around tend to break loose in heavy weather, and boats don't stay afloat if the center of gravity moves too far, so sideways seems like a bad idea. I don't know what the solution is, exactly.