Because they are two separate functions. You can't instantiate a graphics object the normal way, using new Graphics() Or whatever, it must be passed down to you through a function (The draw function, which is the draw wrapper) Or you must extend a class with a Graphics object as one of its members. Actionscript will either call draw or always, but you can't set what will trigger the event.
I'm not quite sure if i'm right tbf. It may be because it's difficult to alter the engine like that, and it's also better this way to keep things separate. But meh.