We still have this notion and hubris that we're above animals, and animals are below us who are alright being stepped on and abused. I noticed that in a lot of cultures, their insults and profanities is being compared to an animal (in Europe, the profanities seem to be generally sexual).
Also, for the religious, admitting we're animals is definitely an insult and denial of biblical teachings that god created humans. When Charles Darwin's theory of evolution first became a mainstream sensation, some cartoonists drew him as a monkey. I debated with a religious before who believes in conspiracy theories. After pointing out about evolution, I was called a monkey. I wasn't even insulted though because, yes, that is basically what I'm trying to say. But technically I'm not a monkey, I'm an ape. Humans are apes. The monkeys are our cousins. Religious folks don't like to admit we're animals because it contradicts their beliefs.