All research has to start somewhere. It is true that parrot pellets did start off vary strongly based on chicken feed. However, with the works of people like Drs. Harrison, Roudybush, Lafeber and more, we have slowly changed them to be more in line with actual parrot needs. Before them parrots ate seeds and died from malnutrition because we didn't know any better. If nothing else, those "chicken feeds" helped buy us more time to find something better. And yes, we are still learning! The food we feed now may be vastly different in 20 years!
Several of the big brands have decades long feeding trials with birds from the 80s still eating their foods.
Food is more than just a sum of it's ingredients. The label on a bag tells you some, but not all, of the story.
People seem to think corn is a bad ingredient. Corn in bird food is a biologically appropriate ingredient. Can you get a pellet without corn? Absolutely. The best part about corn based pellets? They are widely available, cheap to produce and thus buy, and are tasty so our birds actually eat them! Corn based "crappy pellets" have saved the life of my bird, and have extended the lifespans of countless other birds because of their affordability, availability, and palpability!
Food could be grown in the Amazon itself and still be meaningless if the bird doesn't eat it!