Richard, good suggestion and yes, I have thought of that and it is a good means for reaching the email address of the poster quickly, but you simply get a message with an addressee, no subject and no context associated with the discussion posting. I could copy and paste, but that again is more work. In the past I've used a number of different forum tools, and almost all allowed for the "private" response.
The reason for a personal/private reply is to support conversations that sometimes can be controversial and full of emotion. Not always does a posting happen in a manner that fits the direction you had in mind with a learning objective. Yes, I can indicate that in my reply to the student, shared with the class, but sometimes it's simply not appropriate to "call out" or to guide a student while all others in the discussion participate. Instead, I would rather provide some individual insight to guide the student towards a more balanced perception, and then a follow-up posting.
I hope this makes some sense. I teach and work with faculty, as well as students, in higher education and in Business Management / Marketing. We're very focused on critical thinking as a degree outcome. To get to where we think we need to be, challenging discussion questions are the norm. Responses are not always as insightful as we would like them to be and personal guidance is essential as we work to strengthen the learning experience.