In my experience is not so much that they person does not grasp the ethical argument through empathy, many people do. The reason they do not change their behaviour is mostly because it is normalized in society. People do not usually realize it in the moment and so it is not what you would often hear as an answer but uppon reflection, that is what many vegetarians realize—they did understand they were acting wrong but it seemed like it was not a big deal because everyone was doing it.
I hear you-what is the way to resolve this circular trap though? Until a majority of society adopts the argument, many people will continue to mistreat non-human animals, and so long as many people continue to mistreat non-human animals, a majority of society will not have adopted the argument.
I am also in disbelief that people still just stick to tradition or “the norm.”