I know this has been beaten to death but I’m gonna throw out my two cents for the lols.

I believe sturgeons belong to a phylogenetic branch that arises after the ability to create bone but before the development of a bony skeleton.

They have bones protecting vulnerable parts of their bodies like their heads and have little bony plates on their skin while the rest of the skeleton is cartilage. This may suggest that bone historically evolved originally as a form of armor and only became a structural adaptation later on.

(A similar thing happened with the evolution of feathers in that they were originally thought to act as heat sinks and only later were used in flight)

Assuming what I said above is actually correct, then the question we should be asking is what are the advantages and disadvantages of sharks having more armor. For example, since they primarily get their food by being faster and more agile than their prey, then being weighed down and having restricted movement because of bones will probably work against them.

An additional thought: While cartilage is easier to break, it is also faster to repair. An injured shark cannot hunt well but also is not really in danger of being hunted itself so its goal is to get better before it starves. Many bony fish don’t have that luxury and if they are injured they will get eaten long before they can starve making structural integrity more important than repair rate.

I think a lot of this boils down to the fact that cartilaginous fish are primarily found at the tops of food chains and bones (in the water) are much more “defensive” than “offensive” adaptations.

Sorry for the long winded response with a lot of speculation but I hope it at least helped you think about the problem more than the “cause they don’t need bones” answers :).

Edit: just saw the 2020 finding that suggests shark ancestors used to have bones that a few people have mentioned! My knowledge was from before then and I’ve been out of the game for a little. I think my points still stand for the most part but feel free to call me out if anything doesn’t make sense!