At least where I live, in Europe, fish oil is many times less expensive than bivalves, which are priced like luxury food.
Moreover, fish oil is much more convenient, because after I buy a liter, that is enough for more than 3 months, so I have to take care about this only very infrequently.
Bivalves can be stored only frozen and even in that case they occupy a huge volume in comparison with the equivalent amount of fish oil. They would also require additional time each day for preparation and cooking (even if that takes only a few minutes when done in a microwave oven). With cod liver oil or other fish oil, I just add a minute amount of it to the vegetable oil that I add to food anyway, so the extra time for it is just taking the bottle from the refrigerator and putting it back, which adds perhaps ten seconds to the cooking time.
When well prepared, bivalves are very tasty and healthy but they are not an optimal solution for ensuring an adequate daily intake of essential fatty acids, except perhaps in places on marine coasts, close to some farming site, where there might be possible to have continuous access to a guaranteed supply of cheap fresh bivalves.
I would rather not use fish oil, so as I have said, I am waiting for the production process for Schizochytrium oil to become more efficient, so that the premium price over fish oil will decrease enough. When I have tried for the first time Schizochytrium oil, a few years ago, it was at least eight times more expensive than fish oil. The last time when I have checked, it could be found (when searching enough) for as little as three times the price of fish oil (per its DHA+EPA content), so there is hope for a further price reduction.