Go to a fair in farm country and you'll see plenty of variety. But, yes, as with other animals raised for food and dairy (and plants for that matter), mainstream commercial production is mostly a near-monoculture.
After we've all seen what SARS2 can do to the industrial world, the risk monoculture poses for breeding a bird flu which can jump to poultry handlers is simply unacceptable.
Monoculture, and the conditions of chicken husbandry, simply must improve. This is an existential risk.