It's not reasonable to assume that the artists will look at all champions and be inspired equally. They probably don't consider the champion first, they consider the idea first. Then they fit the idea to the champion. If they really wanted to make a skin for a specific champion (like Soraka) they could stare at the champion all day but if there isn't a good skin idea they're not going to make a bad one for the sake of making a skin. This goes with what you were saying about only buying cool skins.