To me, "purebred" means nothing about the quality of the chicken. It just means the offspring will look like the parents. An example I use from an article I saw a while back. A breeder took a flock of chickens and split it. He bred one of the split flocks for large size and the other for small size. The article did not say how many generations it took, but the average size in one flock is now 9 times the size in the other. Both those flocks are purebreeds from the same ancestors.
Unless certain traits are reinforced each generation, chickens quickly can lose those traits. I think that size difference shows the power of selective breeding. This means that you could get the best quality chickens and breed them. If you don't know how to select for the right traits, in a very few generations, your birds would probably be no better show quality than hatchery birds.
Hatcheries are not all alike, any more than breeders are all alike. You get different quality birds from different hatcheries, just like you get different quality birds from different breeders. But you are not likely to get show quality birds from any hatchery, certainly not consistently. Some do a better job than others of picking their breeders, but most use the pen breeding system where they keep many roosters in a pen with many, many hens. The random nature of the breeding keeps genetic diversity up, and most will look a lot like the chicken is supposed to look, but most will have flaws that keep them from being show bird quality. I've gotten the same breeds from different hatcheries and they look different. There are different people with different abilities and different goals picking the breeding flocks at different hatcheries.
When a show quality breeder breeds birds, they often match a specific rooster with one or two specific hens, trying for perfection. Even the best breeders hatch out more chicks that don't make the grade than potential prize winners. It's no surprise that the hatcheries with the randomness of the pen breeding method hatch a lot more rejects.
If you want a chicken that looks pretty much like the breed should but is almost certainly not show quality but will probably lay pretty well for that breed, the hatcheries are into mass producing these birds. If you want a show-quality chicken, you need to find a breeder that knows what they are doing. But they are not mass producing chickens and their prices will reflect that.
