For the same reason why many people in the world experience material privation: not enough time/talent to do it themselves. If you had enough time, effort, ski

For the same reason why many people in the world experience material privation: not enough time/talent to do it themselves. If you had enough time, effort, skill, and raw material, why would you ever need to buy clothes from the GAP, mass produced food from Safeway, or entertainment from BestBuy? Certainly, it would be cheaper and healthier to make your own food. But do you really want to spend all your time making stuff, when, overall, it would distract from what you really want to be doing with your life? In terms of Harry Potter, there is a law of conservation occurring there. Many things 'conjured' there does not last very long, and even to get them into existence requires energy from the summoner. In fact, it specifically mentions in the last book, Harry Potter and the Deathly Hallows, Hermoine states a conservation law about why wizards can't conjure food. (Don't remember which one.) I hope this answers your question.