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Why are cetacean flukes and dorsal fins not made of bone?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Cetacean flukes and dorsal fins are not made of bone because they are composed of connective tissue.1

Unlike the forelimbs, which are modified into flippers containing shortened arm bones, the flukes and dorsal fin serve specialized roles in aquatic locomotion that do not require a rigid skeletal framework. The flukes function as a propulsion surface driven by the animal's powerful back muscles, while the dorsal fin acts as a stabilizing keel to prevent rolling. By utilizing dense connective tissue rather than bone, these structures remain flexible and lightweight, which is highly advantageous for the efficient movement and maneuvering required by these mammals in an aquatic environment.1