I think that it the post apoc conspiracy. I agree with the first comment made by Xenomit. Also, Finn says he is the last person on earth because...he probably is one of the few who are. I think he is insane and maybe in shock and has created this whole world for him.
So he wont be alone and so he can make a new hope. I think each character in the land of ooh *or however you spell it lol* is someone who he knew in his old life and they died. So he re-created them. Like Jake must be his dog, PB could be his crush or girl friend, Marcy could be like a cool rocker chick he knew, Ice King could be his uncle or old guy that was good friends with Marcy?
He just made all of his fantasies come to life and all of him dreams come alive. He had to create his own reality. I defiantly think this is what has happened. It actually makes me feel sorry for him, haha. This could be true or...the creators just have a wild imagination.