A deep, heavy keel can improve stability, but it also takes up weight and space that could otherwise be used for paying cargo. Once a cargo ship is stable enough, adding more ballast is usually less profitable than carrying more goods.
That is why cargo ships tend to have wide, flat bottoms: they maximise cargo capacity for a given draft while generating enough form stability to safely support the load. Smaller boats can rely more heavily on deep keels; commercial ships are built around efficient volume.
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