One reason is that people are simply unaware of both the similar structures of violent ideologies, and the true horrors of animal exploitation.
Another reason is that speciesism has conditioned us to believe that nonhuman animals are “inferior others” whose suffering is fundamentally different from human suffering and whose interests matter less than human interests. For example, although we have evidence that many animals are equally capable of feeling pain, we nevertheless proceed as though humans are the only species that possess sentience. And most people rarely, if ever, question whether humans should have the right to wield complete control over nonhuman animals’ bodies, habitats, lives, and deaths, or whether the tremendous suffering inflicted on billions of animals in order to serve human interests is just.
“Human privilege,” the belief that humans are entitled to use animals for our own ends, causes us to react defensively to the suggestion that humans and nonhumans have an equal capacity to suffer, have an equal desire to live free from harm, have lives that are equally important to them, or deserve equal consideration of their interests. And, like other forms of privilege, human privilege is deeply ingrained, largely invisible, and strongly defended.
A final reason people may take offense at the comparison between human and animal suffering is because they focus on the experience of the victims, rather than on the experience of the perpetrators. It is impossible to know – or therefore to accurately compare – the suffering of two individuals or groups. For example, while there are many similarities, the experience of a Jewish concentration camp prisoner in Germany would have been different than the experience of an African slave in the United States.
However, the mentality that enabled both atrocities was the same; all forms of violence and oppression rely on the same psychological mechanisms.