Because the lesson of the FC App Delivery Membrane is that anything that touches the marginal cost computing power of the cloud must be capital efficient.  Wi-Fi is the most capital-efficient element in the ADM today and, as a result, has sucked so much bandwidth out of networks that most of us hop from Wi-Fi island to Wi-Fi island.  Licensed spectrum, like 4G and 5G, is an unwieldy and immensely expensive bridge between these islands.  Apple wants none of that to get between itself and its customers future applications.