"Outing" people leads directly to harassment. It's happened plenty of times in the past to make a fair assumption that it will happen in the future as well. This is the reality of the situation.
The reason you can't allow it at all is there is no good way to enforce that "outing" someone is appropriate or correct in any given instance or that the person being "outed" is the person attached to said Reddit identity at all.
So that rule protects direct harm from being done on users of Reddit.