It started because people walk into a grow shop without knowing what to do. They buy things based on fancy names and colorful labels, all of which are designed to get your attention and your money. When you combine that with not knowing that some potting soils come with fert already, you get growers who easily mix the soil with the nutrients and over-do it, killing the plants. Hydro ferts tend to require more of the product but that's probably because you're just growing in water only creating a high dilution. Less water is used in a soil or coco grow because these materials hold on to water.