I don't understand their conclusion that no supplement can replace the sardine. They indicate that it's probably the combination of DHA, vitamin D, and calcium

I don't understand their conclusion that no supplement can replace the sardine. They indicate that it's probably the combination of DHA, vitamin D, and calcium, that is good, so just make a supplement containing this combination? And if there are missing ingredients to the magic of sardine, just add them too? I don't see what's impossible, except that they want to support their country's fishing industry.