Harrison’s and Nekton formulas have been very carefully calculated and created by avian nutritionists and avian veterinarians. Ingredients and ingredient amounts are chosen based solely on the intentions of balancing out a formulation that will yield excellent results.
The difficultly with seed is that it is incomplete with everything the bird needs in a diet. This proves to be bad if parrots are given a seed only diet as seeds are often high in fat. Usually what birds do is that they pick out exactly what they like (the fatty seed) and leave what they don’t want behind. With high fats and over eating accompanied by the deficient vitamins, minerals, and amino acids, birds set on a seed only diet will not develop and grow as they should. Formulated diets are also 100% edible and no wastage which is a great advantage over seed as seed has about 40% wastage.
If you make chop, sprout your own seeds, grow your own veggies, and feed all these to your birds, your parrot will not starve. But if they pick and choose what they eat, they may not get the full range of nutrients they require. Pet parrots also do not exercise as much as they would if they lived in the wild. All of mine are flighted, yet they choose to walk everywhere. I too am capable of running, but choose not to - it's too much work! And because of that I can't have my McDonald's KiwiBurger every day of the week, even though technically it has all the 'good foods' - meat, bread, egg, and beetroot! Seeds are McDonald's for birds. It's not the seeds themselves, but how they're delivered and eaten, and the high fat content. Harrison's does contain seeds but it is formulated and combined in such a way that your bird gets all the nutrition at the same time as the seed taste they enjoy. Harrison’s does not recommend peanuts or sunflower seeds when free-feeding, but they are included in the pellet formulation because there they are balanced with other grains, vegetables, and vitamins for a complete diet.
A lot of bigger parrots like to hold onto their food while they eat, pellets are a great delivery method that seed just can’t provide.