I would instead ask, why has God always used humans? Everyone knows that He could have done it all Himself. We understand and believe that there is an omnipotent God, but as humans we relate with and better understand relationships with humans. If you were truly knowledgable about Salvation History you would know that Mary always leads people to her Son. That God used her as a conduit that humans can understand and relate to.
I believe that you are sidestepping the specifics of my question by responding the way you have. Your comments have very little to do with the issue, which is that of Mary appearing as a pregnant woman. Of course God uses humans. The point is that in this particular message, there is no glorification of Jesus (aside from the single reference to the Almighty, and that doesn't even address Jesus by name) but rather it appears to glorify Mary. I know that is the standard reaction from non-catholics when looking at this kind of thing but honestly I can't interpret it any other way.