The scarce thing is being able to say "Cryptokitty #123 belongs to me". No one else can say that, and everyone will agree that you're the answer to the question "Who owns Cryptokitty #123?" or "Who owns the Cryptokitty with that specific appearance?". It's definitely a bit silly, but I still feel there's some neat allure to that. Maybe try thinking of it like owning the original piece of some fine art. Art can be photocopied, so what's the point? There's some kind of prestige in having the original, in knowing that you have a direct relationship with where it came from, and the fact that your direct relationship with it may affect others who are interested in the art.
And about the specific cat images themselves: sure, anyone can create a cat image like these, but there's a certain fun in knowing that the image was produced by a specific system and by people learning and using the rules of the system. Anyone can take a chess board and its pieces and arrange the pieces into a winning configuration, but that's missing the social significance that you get from playing a game against someone and getting the pieces into that winning position through artful play within the game's system of rules.
And about the specific cat images themselves: sure, anyone can create a cat image like these, but there's a certain fun in knowing that the image was produced by a specific system and by people learning and using the rules of the system. Anyone can take a chess board and its pieces and arrange the pieces into a winning configuration, but that's missing the social significance that you get from playing a game against someone and getting the pieces into that winning position through artful play within the game's system of rules.