Pretty simple, human empathy is plastic, and many of us are conditioned to extend our nurturing instinct to our pets. But we are also tribal carnivores, so our empathy has limits - out group violence is “ok”.
I know people who have very broad empathy, even to insects. And that kind of empathy can be painful and debilitating given how much pain and suffering there is in just the natural world of animals, let alone the way humans treat animals.
