Let's just say there isn't enough "usable" spectrum, which partly has resulted in this licensing regime. Different frequency signals have different propagation

Let's just say there isn't enough "usable" spectrum, which partly has resulted in this licensing regime. Different frequency signals have different propagation characteristics and so only certain frequency ranges are amenable to specific use cases. For one, higher frequencies are more easily absorbed by materials, so you only get a few 100 MHz of frequencies suitable for long distance transmissions. As an example, even for home WiFi, you will note that 2.4GHz signals will travel farther in your home than 5GHz signals. With today's tech, usable frequencies are a scarce resource.