They are half-duplex because all parties share the same frequency. Only one party can transmit at any time.

Full duplex would require two frequencies and therefore third parties would have to monitor both frequencies simultaneously in order to hear both sides of the conversation. Half-duplex operation on a single frequency is inherently 'open-channel' - everybody can hear everything transmitted by anybody else on that frequency. That's the way it's always been done in aviation and in many other fields.

(Answer taken from comments by https://aviation.stackexchange.com/users/13124/transistor and https://aviation.stackexchange.com/users/15152/fiddlesticks)