Radio spectrum is a shared resource that for many use cases needs exclusive allocation to work well - if one person/organization is using a frequency band in an area, they need to be the only ones who can use that frequency band there. Think TV or radio stations; if someone else is transmitting in the same channel, then it just disrupts the broadcast. So most such spectrum is allocated (often through auction) and then whoever has that allocation is legally granted a monopoly on the use of that spectrum, enforced by government who will prevent anyone else from using that spectrum with fines and even physical interference (finding, disconnecting and confiscating the violating transmitters) if needed.

Transmitting over radio waves is a highly regulated privilege. There are a few ranges that are open to public (i.e. the bands used for Wifi and BT) with devices that need to be certified to ensure that they transmit only in the narrow permitted ranges, but in most frequency ranges (including the LTE/5G ranges) it is illegal to operate a transmitter without an explicit license.