A Baby Monitor with Sound Cancellation

I've mentioned the physics behind noise cancellation before.  It's pretty simple, sound is energy in wave form and any wave that is met by a wave of equal but opposite magnitude will cancel out.  Luxury cars have this system to deaden the noise getting into the cabin.  The car "listens" for noise and then the speakers emit a sound of equal but opposite magnitude.  Why not incorporate this technology into the standard baby monitor?  Then when the older kids are going crazy or the dog barks or the doorbell rings you could rest easy knowing the sleeping baby's room was sound buffered.