Why do Tom and Huck have to make up so many of their own difficulties when helping Jim escape?

Tom sees the escape as a game, thus, to create as fantastical a story and game as possible, he tries to determine how to make Jim into a real prisoner before his daring escape. He decides that he and Huck will have to saw off the leg of Jim's bed in order to free the chain, send him a knotted ladder made of sheets, give him a shirt to keep a journal on, and get him some tin plates to write messages on and throw out the window. To top it off, Tom tells Huck that they will use case-knives to dig Jim out, rather than the much quicker and more appropriate picks and shovels.