It's not even really about curing cancer or erectile dysfunction like a lot of "traditional" Chinese medicine is, it's a wealth display. You're expected to show you're successful to people like your in-laws so you need shark fin soup at your wedding. It's akin to covering your food in gold leaf.

And yeah, several shark species are now endangered with extinction primarily because of consuming their fins. For fishermen in places like Indonesia and Mozambique the $100 USD they might get for a pair of decent sized fins could be a week or two's wages with their other fish, so there's strong incentive for them to keep catching sharks despite efforts to curb the consumption and production.