Here's why:
* They find it pointless and unnecessary. Jem and Scout are used to learning by experiencing the world around them, through their own exploration and the stories told by their father, Atticus. Miss Caroline's rigid, structured approach feels artificial and irrelevant to their way of life.
* They are skeptical of its effectiveness. Jem, especially, sees the Dewey Decimal system as a way to "memorize" facts without truly understanding them. He believes it's a way to "choke" the life out of knowledge, rather than letting it flourish.
* They struggle to relate it to their own knowledge and experiences. The system seems foreign and confusing to them. For example, when Miss Caroline criticizes Scout for knowing how to read, it highlights how the rigid system doesn't account for individual learning styles and existing knowledge.
Jem and Scout's reaction to the Dewey Decimal system highlights the novel's theme of the clash between formal education and real-life experiences. It also foreshadows the difficulties Scout faces in navigating the social and educational world of Maycomb, a world that often feels disconnected from her own understanding.