Generally, I don’t take supplements. But when I do, I buy them in their pure, powdered form and encapsulate them myself.
A supplement company does the same thing, only on a larger scale. It acquires pure, powdered supplements from one of the few supplement manufacturers and packages them into dosage units (e.g. capsules or tablets). Then, it bottles those dosage units into containers that bear the name of its company, before selling them, at a premium, to wholesalers and retailers.
Not only is doing it on your own cheaper, but it also allows you, the user, to decide on the type of dilutant[*] to use, as well as to avoid certain excipients that may be necessary on a large production scale, but unnecessary (and possibly allergenic) for making supplements at home for your own use. You also get to create your own unique mixtures of supplements, and to vary the proportions of the supplements in those mixtures – handy for self-experimentation purposes.[†] And, lest the requirement for comprehensiveness be disregarded, some powdered supplements taste awful, so encapsulating them automatically gets around this taste factor.