Because wizardry is like life in that universe, and while you personally could learn how to manufacture a cell phone, you'd still need significant resources, equipment, insider knowledge/private IP (Know the spell/incantation), and time. This parallels our world where it might be easier to enchant specific items you've specialized in to sell (crafting, maybe like a self warming tea cup) or work in magical remedies/restrictions/enforcement (service work) for currency which you can then use to purchase a wand than it would be to harvest the ingredients for your own wand and somehow know the enchantments and crafting techniques to create it. Also, wizards are magic, they are not all powerful.