Because outputs are triggered by the router itself, not by external factors. For example, inputs are triggered by external devices, so there is the possibility to configure rules that execute certain actions on that event. Output states, on the other hand, are changed either manually by the user or by some automatic configuration (like input rules).
Is there a particular reason why you need events reporting on output state changes? I'm asking because the function isn't there because there's never been any need for it.