IDK, nobody ever held a gun to my head and forced me to buy anything whenever I went to a "hydro" or grow store. Consumer ignorance can lead to issues in a variety of situations.
There are still different ways to grow and everybody has a different set of circumstances to grow in. Not to mention, the same companies that manufacture/package or distribute premixed gardening products are also packaging/distributing raw ingredients and amendments.
Premier Tech packages Premier Peat Moss, various Pro-Mix peat and coir products. Sun Gro Horticulture makes Sunshine Mix, Sunshine Advanced, and Black Gold potting soil/mixes and soil amendments/conditioners/fertilizers. Even Fox Farm Soil and Fertilizer Company packages/distributes amendments and dry all natural fertilizer mixes, and potting soils; aside from manufacturing hydro/chemical products... and BWGS(Bloomingdale Wholesale Garden Supply) distributes everything under the sun. Not just to hydro/grow stores that sell to pot growers, but also to greenhouses, nurseries and garden supply centers.