So I was often told – but not everyone in a choir follows this rule.
Singing develops certain skills which may have their uses outside performance. One is the ability to control volume precisely, and this can allow someone to say things audible to those around them in rehearsal, but out of earshot of the conductor! Many a choir practice in Manchester was enlivened by the dry comments of one of the singers just behind me. (I can’t remember any of them now, and without the context they probably wouldn’t be very amusing).
A less entertaining characteristic of singers is that almost by definition we like the sound of our own voices. This can have long-winded consequences at those evenings we all dread – the choir AGM.