A human at the other end of the exchange forces you to think about that human, to consider that human, and to acknowledge the existence of that human, whether they speak or not. Don't have to feel that way with a machine.
Yes, some people are that bad that even silent interaction produces anxiety. It's why I prefer emails and texts instead of live phone calls. I can communicate on my own time after thinking it through and not feel obligated to respond immediately (that's what Asperger's does to a MF)