At a distance, the notion seems appealing. But when you try to build anything larger than a simple form UI, you start to realize that maybe it's not such a good idea to have anyone, anywhere, listening to changes on any old property and doing who knows what.
There are two problems with automatic change notifications: timing and context. Usually there is a something else (e.g. a controller) that is actually doing the property change on a model object. This controller is much better positioned to inform others about the change than the model itself. The controller knows the context, and it can decide whether the notification should go out instantly or perhaps after a bunch of other stuff. The controller can also make this context information available to the listeners, which is often invaluable... But a plain old "this key just changed, ho hum" doesn't let you do any of that.