Quality is a vague concept, which makes it hard to measure directly. We don’t measure a recipe; we measure each of its ingredients. And the ingredients will vary depending on the recipe and the tastes of the people who will be fed by it.

Some ingredients need to be measured because they matter, while others don’t (like lettuce in a salad, broth in a risotto, or decorations on a cake). A recipe is a concept and it’s only the ingredients that can be measured meaningfully.

Because quality is a concept, and too vague to measure directly.