The shape is the result of the way the ships were built. They started with the outside planking of the bottom. Next one horizontal floor was placed over the planks, with on both sides a curved part, the first futtock. Then the bilge was planked in the same provisional way. As the floor was flat, the bilge necessarily formed an angle with it.
These ships were built for loading capacity only. Speed was no issue. The stuff they transported was not likely to rot. Draught was another item, because Dutch harbors were shallow. The result was a reasonably stable ship type.