Is because of the fact that sharks skeletons just like fish skeletons are primarily composed of cartilage, not bone. The only thing made of bone is the teeth.
The sentiment is right (shark skeletons are made up of cartilage, which is a soft tissue, unlike bones, which are largely calcium phosphate; the only bony part of a shark is the tooth). The only problem is that most fish species are actually bony. Of the roughly 28,000 known species of "fish", about 27,000 are bony. Another ~970 are sharks, rays, and chimeras, and the rest being lampreys and hagfish.
Fish is in quotation because apparently, the class Pisces is out of favor, and fish are now defined as any non-tetrapod chordate that has gills throughout its life, and if it has limbs, they are shaped as fins.
Fish - Wikipedia
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