Many people take cod liver oil, but what is it about a cod's liver or lifestyle in a particular that makes it store up the nutrients etc that people want to take (omega-3, vitamins etc). Why don't other animals do the same?

interesting question!

the link below describes the components and how the capsules are made. The important point to note is that most if not all fish livers contain similar beneficial substances. Other examples are shark or halibut. the reason cod is most commonly used is (I suspect) ease of production and cost.

https://en.wikipedia.org/wiki/Cod_liver_oil