Beyond believing that it would be disrespectful to and unhelpful for clients (which I do believe), there are other very good reasons for therapists not to do it.

If I knew information about a client that they hadn't told me themselves, I would find that awkward and difficult and it would interfere with my work with them in a way that I would not appreciate. So it's also for my own benefit that I don't do it. I've been curious before, but it's not that difficult to refrain.