I think it’s probably because they see the racehorse alive, in all his glory, on their television screens. Then his life is suddenly “cut short” by “greedy humans.”
Meanwhile, they never saw that chicken in cellophane alive. People are either completely disconnected from it as a living being, or they lie to themselves (often without cognizance of doing so). They fool themselves into thinking it lived like those happy chickens they saw roaming free range on a third grade field trip, then died a peaceful, natural death in order to become your McNuggets… and don’t think about it any more than that.
I’ve also noticed how different awareness is for “animal people” v. “non-animal people.” Even just driving down the road: every single one of us would notice that chicken truck next to us at the stoplight and probably have some sort of reaction. Non-animal people don’t even see it or acknowledge it. If it is brought to their attention, they certainly don’t give it much deep thought.