They are as purebred as the hatchery wishes to make them.
There is a bit of confusion as to "purebred" and what that means as to hatchery versus breeder stock birds. Real breeders, not just some propagators or chick mill, but true, faithful, reputable RIR breeders of Mohawk Line, let's say, birds are quite rare. These are bred to true type, to standard, to SOP quality and may or may not be shown, but are absolutely gorgeous, heritage birds. A very limited number of them are raised each year. They are quite hard to come by. There isn't nearly enough of these birds to go around as there are people who might want them. They are also only usually hatched for a limited amount of time, each spring. Gotta get on a waiting list.
This is true for the other breeds as well.
But not everyone needs this quality of bird. Unless you are going to breed them, show them or be a part of the heritage preservation minded world, there's no need to have them. Heritage birds aren't for everyone. They can be quite expensive. They take forever and a day to grow out to size. This all takes feed, and more feed. Being true to their breed, they will often lay no where near the eggs of a hatchery stock bird. A heritage fowl will often go broody.
The hatcheries mass produce birds for profit. There is no matching up this cock with that hen for excellent results. Just mass production. A hatchery bird is said to "represent" the breed. They mature faster, they lay more eggs, as they've been bred for higher egg production. They aren't true to body type, feather color and rarely go broody as that isn't a trait that a hatchery can profit by. A hen that is setting isn't producing eggs.
Hatcheries provide a service, as they sell birds cheap, quick and almost year around. That's what they do well. Their birds are normally healthy and provide an adequate chicken for people.
I hope that helps.
