I have a hypothesis. I believe that fictionkin who aren't kin for the sake of coping or anything really want to be famous without putting effort into it. Like, imagine having a huge fanbase and a popular piece of media starring you. Who wouldn't want that? These people turn that fantasy into a reality, and if that means taking the identity of a fictional character, so be it. Meanwhile they placebo themselves to the point where they get memories and other things that come with being a fictionkin.