Because while running the tranny and pumping fluid out at the sametime, you run the great risk of burning your clutch packs or just plain causeing undo wear on the components of the tranny.
Yeah, it could be done that way. I'm not saying that it can't, but I personally feel that the risk is too great. I will add that the best option is performing a flush on the tranny, but in cases where the fluid has never been changed in longlonglong time, that could do more harm than good.
