Why is networking organized into layers at all?

Because the problems networking solves, physical signaling, addressing, routing, reliable delivery, encryption, message format, are too varied to handle as one monolith, so they're separated into layers that each do one job and interact only with the layers directly above and below through defined interfaces. The benefit is modularity and independent evolution: because each layer is a contract, you can replace the link layer (swap Ethernet for WiFi) without changing IP or TCP, improve TCP's congestion control without touching applications, or run the same HTTP over completely different physical media. It also enables interoperability, different vendors implement the same layer to the same spec and interwork, and divides the engineering so a router team thinks about IP while a NIC team thinks about the link. The cost is some overhead (each layer adds a header) and occasional inefficiency from strict separation, but the manageability and flexibility are worth it, which is why every real network stack is layered.