That is because the image and the post are treat as separate entities, with the post providing metadata for the image so the search engine will find it when you query. Even from Reddit's sure, if the post is gone the image may remain so you could be being served that by Reddit (or third party) rather than from Google's cache (any thumbnail will come from Google of course).
E.g. images deleted from Reddit can sometimes be seen in Google image results, but when following the links they might lead to deleted posts.