The birds being grown for meat or for laying eggs do not have good enough genetics to be used as breeding stock for future generations.

You can imagine a family tree containing every chicken used for large scale meat/egg production.

At the top of the family tree is a very small flock of pure bred master breeders. These live in a lab environment and are guarded 24/7. These birds have been very carefully selected to have the purest genetics and it’s been an ongoing process for the last 30 years. You can’t find a more perfect bird anywhere else. That small flock is worth hundreds of millions of dollars.

At the bottom of the family tree are the common birds that are being grown for meat and laying eggs.

There are 8 or 9 levels of family tree from top to bottom. Each level multiplies the population by as much as 200x, but at each level down the genetics worsen.

The bottom layer of the family tree’s population is about 60x the population of the previous layer. The bottom layers population is many times the population of all the other layers combined. Due to statistics it’s almost always the bottom layer of the tree that gets hit with avian influenza. Those birds’ genetics are so diluted they are not worthy of spreading their genetics to future generations. It would take 30 years of breeding to purify the genetics to take out all the tendency for splay legs and imperfect breast meat and all the other things that are selectively bred for.

Sometimes a second from the bottom flock gets avian flu. That can really mess up the whole system. For example say you lose a broiler breeder barn with 25,000 birds. Losing that one barn of 25,000 birds has a trickle down effect that will result in 5 million broilers not being grown. That broiler breeder barn could have produced 25,000 broiler chicks a day, and that barn will be off-line for 28 weeks (how long it takes to grow a broiler breeder flock to breeding age).