Why only three phases?

Twenty years ago, broilers were marketed at eight weeks of age. Feeding them was done using three diets: starter, grower, finisher. Today, broilers are marketed at six weeks of age (at similar weight as those marketed at eight weeks of age mostly due to genetic advancements).   We still feed them three diets: starter, grower, finisher. The diets are a bit different (more concentrated) and the time each diet fed is also different (shorter) than before, but we still use three diets. Why? Why n