Under "ideal" conditions you would be correct but most of us lead busy lives and it's just not "practical" to make it by the drink.
Personaly I also think doing it bulk helps many stay on track and stay on the cleanse by having the drink readily available every time they feel hunger or need an extra energy boost or gets a head ache etc. If someone is teetering on quiting. I think if the had to make the drink first each time they would be more likely to quit