How are you so sure they don't grieve? They form social relationships, they defend each other, they groom each other, they cooperate. They have complex vocalisations, they warn each other even when they themselves aren't in danger.

Why are you so confident they feel nothing when a friend dies? why are you so confident they don't feel fear as you kill them?

https://www.chickenfans.com/do-chickens-mourn/

When two chickens are close, they'll mourn each other's death.

https://backyardfarmlife.com/is-your-chicken-sad/

When it comes to the capability of chickens’ emotions, it’s known that chickens experience friendship within the flock, experience grief at the loss of a fellow chicken, and chickens can even miss their owners.

https://bestfarmanimals.com/why-is-my-chicken-dying-everything-you-need-to-know/

There are instances when chickens bond closely and grieve when one of them dies.

https://backyardhomesteadhq.com/chicken-feelings-do-they-miss-each-other/

Chickens are thought not to understand the permanence of death, and so not to be able to grieve . However, they do display some empathetic awareness of when another chicken is sick or dying.

You wouldn't believe how quickly I found all that! The other thing is: if you applied that logic to people, it becomes problematic very quickly, and there's nothing about animals that precludes them from deserving the same hesitancy around those problems.