The higher protein content of the starter/grower is because the young bird's growing body needs the extra protein levels to build their growing bodies. Once the birds have reached the point that their growth slows their need for protein lessens so the commercial feeds drop that level. This is similar to how the overall need for protein, fats, etc fluctuates throughout the life of a human based on what the body is doing at a particular life stage (ie infants/children with growing bodies needing a different nutrient balance than an adult with a stablized body vs. an elderly person with a body that is in the process of shutting down) That being said, plenty of people *do* feed the higher protein "flock raiser" type feeds to all of their birds for their entire life cycle - the advantage of doing this is that you don't have to be as concerned about the supplemental feeds (scratch, food scraps, etc) that are lower in protein content causing too much of a drop in the overall protein level of the diet as a whole that would bring the actual protein content of the bird's diet below even the 16% level of the feeds prepared for mature birds. Plenty of folks also offer meats as part of their birds' diet.
