Those are culls, not slaughters. They aren't the same thing. You cannot sell culled chickens for food, they are culled due to disease and are considered biohazard, because they can infect other parts of the flock or other flocks, and in the case of bird flu are also infectious to humans. Generally, the USDA requires a full cull of flocks if any birds are affected. My sister is into urban chickens and bird flu is very much not a joke, and trying to prevent it is hard because humans can carry it and infect chickens and chickens can infect humans.

Also, the chicken you eat and the chicken that lays eggs aren't even the same breed at industrial scale.

Also, I know that many regulations, including USDA regulations, are a bit of a joke in the US, but bird flu is one thing they very much do not fuck around on. The loss of those 60 millions birds is a straight economic loss that's non-recoverable for those egg producers.