then it would force them to actually confront the morality of themselves eating meat.

For many people, "I am a good person" is a core belief. An emotionally strong person may be able to look at a challenge to their core beliefs, assess its validity, and adjust. Most people are not emotionally strong. They see this thing like vegetarianism as an attack on their core self. And since they are not emotionally sound people, they lash out or make excuses. That's easier than accepting maybe they're not flawless paragons of virtue.

The oatmeal did a comic about this: https://theoatmeal.com/comics/believe

D&D webcomic "the order of the stick" also did a comic that touches on it. One of the characters says that people like this, who refuse to look in the mirror and honestly assess maybe they've done something bad, are cowards. https://www.giantitp.com/comics/oots1151.html

And honestly that's kind of it. People are scared, and scared people are stupid. The big "i eat nothing but MEAT" people are probably, contrary to their posturing, cowards. Worse than children, because they'll probably never grow up.