If you ever tried crafting in 1.0, where everything was interconnected and nothing was sold on the merchants, you'd understand why.
Moreover, the low level crafted items like hempen yard, hempen cloth, etc. cost more to make than they'd actually be worth.
nobody wants to go out and farm hundreds of low level items to make items they'll never use. In addition, having certain items like iron and copper for sale, items that are used in large quantities for higher level recipes, is an enormous time saver.
Plus it makes sense. Why wouldn't a crafter's guild have materials on hand to get their new members started?