This is a tricky bugger for us. Cognitive dissonance occurs when our brains have conflicting ideas, which usually causes us to construct bull shit backdoor escape to avoid the conflict of ideas present in our minds. Alright, sometimes this escape is logical, or at least seems so; the point is that we are completely invested in finding some coherence in our currently held beliefs, rather than trying to find the better ones.
Simply put, cognitive dissonance (fueled by desire and tradition) is the greatest obstacle to human beings, in a general sense, treating other animals as if their lives and interests are worth anything at all. That may seem harsh, but it is a simple fact that an omnivorous human will trade the most trivial interests of their own for some any, including the most integral, interests of other animals. There are a few exceptions to that, but they grow less and less every day. Anyone who can read this and type a response can become vegan, which just means to give a shit at all.