it’s defensiveness. a person who eats 19 strips of bacon for every meal doesn’t threaten the average omnivore. that person is arguing that they should do more of what they want to do anyway. the existence of a happy, healthy vegan, OTOH, threatens omnivores. it tells them that there is a choice other than meat, and what that does is force them to acknowledge that eating meat is a choice and that if you make that choice you’re responsible for the consequences. if you live in a world where meat is necessary, let’s call it the ferret diet because they’re my favorite obligate carnivores, then you didn’t really have a choice at all. as factory farming imposes cruelty on animals at the individual level and huge damage to the environment and climate on a collective level, the ferret diet allows you to say “🤷🤷 what are you gonna do?” veganism is an attempt to answer that question, and it’s a valid one. there are plenty of people who don’t eat any sort of animal product and are still happy and healthy. veganism threatens them because it makes the suffering they create a choice that they’ve made, rather than an inevitable consequence of being an obligate omnivore. bitching at vegans, trying to poke holes in vegan diets, all it is is an attempt to shed responsibility for your own life choices by pretending there never was a choice.