Not sure if you're joking - but vegetarians / vegans tend not use use any product which is made by killing an animal.
People can argue about whether shearing a sheep or obtaining shellac is harmful - but I think everyone can agree that there's no way to extract the liver of a shark without killing it.
Virtually every product is made by killing animals. I'd say vegetarians/vegans only really care about products that ~require~ the killing of an animal, and care very little about the products that only kill animals incidentally.
It's still human dominion over animals. And veganism is still veganism. e.g., don't tell me that "some vegans decide it's OK to wear leather" because they're not "a collective entity" or something like that. Veganism is very much a black & white ethos. There is no acceptable gray area for veganism.
Maybe not (unless you want to get at that 'roid from the long way around) but some people are vegetarian because they want to avoid killing animals as much as possible. It seems that shark liver oil comes mainly from sharks already considered "vulnerable" by the IUCN[1]. I'd feel weird about rubbing the liver oil of a top tier predator on my ass now that I think about it.
People can argue about whether shearing a sheep or obtaining shellac is harmful - but I think everyone can agree that there's no way to extract the liver of a shark without killing it.