Because those are perishable products and I basically never just happen to have the proper cheese in the proper quantities for macaroni and cheese in the house.

Because those are perishable products and I basically never just happen to have the proper cheese in the proper quantities for macaroni and cheese in the house. If I wanted to make it "from scratch" I'd have to plan ahead; my day-to-day diet has very little cheese in it. Cheese powder, on the other hand, is non-perishable so it can always be kept on hand. It's also MUCH cheaper.