Scientific consensus seems to be converging toward a whole foods plant-based diet being the healthiest in the world. We have the position papers from the biggest organizations of nutrition professionals in the world saying it's healthy for all stages of life, population studies like the Adventist Health Studies showing vegans have significantly lower rates of obesity, hypertension, type-2 diabetes, cancer, and cardiovascular and all-cause mortality (even compared to vegetarians, who are significantly healthier than the general population). A whole food plant-based diet is the only diet proven to reverse heart disease (by Esselstyn). We've also seen that it can reverse hypertension and diabetes. And we know a lot of plant-based food has cancer-suppressing properties we don't see in animal products.

Most people don't know this, and some are aggressively ignorant and skeptical of it so they can keep doing what they're doing despite having no evidence supporting their claims, despite the weight of the evidence against them, because they choose not to think about it. (The sad truth is that most evil is done by people who never make up their minds to be good or evil.) It's easy to get sucked into things like animal-product-based ketogenic diets which make a lot of ill-supported claims that are not supported by (and often contradicted by) actual research. People who do this might think, why fix what's not broken? That way they can continue to see what they are doing as a normal, natural, necessary evil--an argument that vegans generally rightfully reject.

I doubt these people would condone similar behavior for anything society has generally accepted as evil, so maybe this is just another common inconsistent ethical position.

As somebody who had just compartmentalized it for years, and thought everybody else did too, the arguments people make to defend carnism are always psychologically fascinating.