That's since the mobile unit and the base station both need circuitry to transmit on one frequency while receiving on another. But it's the only way that permits a normal, back and forth, talk when you want to, conversation. Take a look at the animated .gif below to visualize full duplex communication. Full duplex communication example. The two frequencies are paired and constitute a voice channel.
Paths indicate direction of flow.
The mobile switch assigns which of those channels to use for a given call, but has no ability to assign other channels. This takes several hours. An engineer may work for weeks on a frequency plan just to add channels to one sector. Mark van der Hoek. A good but flawed book that's now in its second edition.