Cats arent actually supposed to eat fish; as the species was moved around the planet, they started having to eat fish, because certain cultures only had that to feed them, but in short it doesnt provide the necessary nutrients a kitty cat needs and isnt all that healthy for them. They like it because they like the taste, just like dogs enjoy chocolate, or most people enjoy deep fried foods.
Done ranting :3
*gives tuna with a belly full of neko*
*insert corn*