Anyone may start a website, but not a radio or television station. The difference is physical: the airwaves can only carry so many signals at once.
Interference is the underlying constraint
Two transmitters using the same frequency within range of each other produce interference that renders both unintelligible to listeners.
There is no technical workaround at the receiver, so the problem must be solved by controlling who transmits on which frequency and at what power.
Licensing is that control. It exists to prevent a shared physical resource from being rendered useless by uncoordinated use, and it works by making transmission conditional rather than open.
The constraint is genuinely physical rather than administrative. Adding capacity to a printing operation requires only more paper and presses, while adding a station to a full band requires taking capacity from something else.
Allocation happens at several levels
Bands are first divided internationally, since radio signals cross borders and neighbouring countries must agree on which uses occupy which frequencies.
National regulators then assign specific frequencies to specific transmitters at specific locations, with power limits and antenna characteristics attached.
Geography does much of the work, since the same frequency can be reused in distant markets where the signals will not reach each other. How far apart those markets must be depends on transmitter power and terrain.
Coordination between adjacent markets is therefore continuous rather than settled once. A power increase requested by one station can affect reception hundreds of miles away and must be assessed before it is permitted.
Licences carry obligations
Because spectrum is a public resource, licences typically come with conditions rather than being simple property.
These commonly include requirements to operate as authorised, to identify the station, and in some cases to serve the local community in defined ways.
Renewal is periodic, and the possibility of conditions being reviewed is what gives regulators continuing leverage over how the licence is used.
Scarcity shaped broadcast regulation
Courts and legislators historically justified regulating broadcast content more heavily than print precisely on the grounds that frequencies were scarce.
Because only a few could broadcast, the argument ran, those few carried obligations that a printer with unlimited paper did not.
Cable, satellite and internet distribution have weakened that reasoning considerably, and the appropriate scope of broadcast regulation is now genuinely contested.
Efficiency created new capacity
Digital transmission fits more content into the same bandwidth than analogue did, allowing a single licensed channel to carry several programme streams.
The transition also freed blocks of spectrum that were reallocated to other uses, most visibly mobile communications, through regulatory processes and auctions.
Scarcity was therefore reduced rather than eliminated, and the competition for spectrum now runs between entirely different industries rather than between broadcasters alone.