Therapists who practice psychoanalysis may present themselves as a blank slate in order to encourage transference in client. Most of the time, however, I think Ts do this in order to keep their own "stuff" out of the therapy room. In order to bond with clients Ts might share select information (marital status, children, etc), but most keep it to a minimum. Anything personal a T shares should somehow benefit the client, otherwise it can detract from therapy. That said, I think some Ts have difficulty distinguishing between what is and might not be appropriate info to disclose. It varies so much from client to client that the room for error is huge, so I think a lot of Ts opt to stay neutral to avoid screwing up. They err on the side of caution.