Because there is no substance to the food, nothing to stimulate digestion/metabolism, and it's in and out of you very quickly, even w/an EFA in there to "slow" digestion.
Try it and see how it works, that's all that you really can do so that you may see what all of us are referring to. We can give you 100000 reasons why it's not a good idea, but only you can experience it for yourself.
Maybe that's why I haven't gotten much bigger lately...I'd like to know some of the other reasons why, though. If you're willing to tell me, of course.