To cite a close-to-home example, chicken farms in Canada typically have about 25,000 chickens, whereas ones in the U.S. often have millions. So an infection that requires the entire flock to be slaughtered has a much bigger effect on the supply of eggs south of the border.
That makes a lot of sense, because I lookup up how we handle it in Denmark and it's the same, destroy the entire flock if a farm is infected. It's just it's not millions, it's 6000, 40.000, 20.000 chickens per farm, not a million.
Weird that the size of the farms aren't being regulated if you know from other countries that it makes containment easier.
Unless you segment up your chickens and spread them out, so one farmer may have a million chickens, but spread out on 40 locations. The problem is that you need to kill ALL of your chickens in just a few is sick and having a million chickens in a single location will pretty ensure that you have to constantly kill of all your chickens and replace them.
But there's probably more going on that just sick chickens being killed of.