probably the same reason they dont buy a couple of ingots of silver and suspend them in distilled water and make Colloidal Silver for pennies a gallon. or buy their own custom blend of herbs in bulk and make their own capsules. both of those i do myself, among many others.
but most people want to eliminate as much chance and trial and error as possible. people are always asking here about dosage. their goal is not to experiment but to address a problem, usually.
people buy from me because they trust me. they know they are getting exactly what i say they are getting. they have access to me with any questions.
i am a combination convenience store and personal laboratory. there is a high degree of probability that at any given time i know exactly what i am talking about. plus, people like me.