I don't know if this is the reason but it'd take a lot longer to fill and stack then unstack and empty 200 containers than it does to fill one cavernous space.
Then you still have to clean those containers just like they are cleaning the ship.
I was thinking filling it would be faster, but unloading the sand they way they did, scoop by scoop, seemed like it’d be slower.
Also, I was thinking that if they used containers they wouldn’t need to clean the ship (or at least not to food grade) since each cargo would be in its own compartment.
The shipping industry offers a transportation service to its customers. In the bulk area, not leaving your customers to clean hundreds of containers is part of the service.
Then you still have to clean those containers just like they are cleaning the ship.