Why not just vaccinate? Vaccinating flocks would, according to current trade agreements, prohibit export to many countries and thereby preclude huge earnings from the $5 billion poultry export sector. Vaccination is, therefore, prohibited.
What this means is that many flocks are probably culled despite being predominantly healthy. It also means that producers are incentivized to raise as many animals as possible in the ways that they always have.
Meanwhile, the U.S. poultry industry receives billions in annual subsidies.