The term "pure bred" is a misnomer of sorts, when it comes to chickens. Chickens are better described as bred to the Standard or true bred. Breeds aren't species, they're composites of many bird types, put together a century ago and a Standard was then written and accepted.
Birds truly standard bred just about must come from a breeder, someone passionately dedicated to the preservation and presentation of the breed. Hatcheries are businesses and must virtually mass produce birds. The birds are healthy, cheap, widely available either through mail order or through secondary feed store re-sellers. These birds are bred to be healthy and productive, often very productive.
Bred to the standard? No. That's just not in the business model. It's quite understandable.