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.

wait, I can eat bacon for every meal and that would be considered a "diet"?

I'm no nutritionist, but I'm reasonably sure that any reasonable diet, whether keto or vegan can be accomplished while maintaining proper nourishment.

The thing is, most people's diet isn't even providing full nourishment. There's usually something that's missing that people are simply not aware is missing, or they're getting in such low quantities that it's unhealthy. IMO, the main problem is a lack of education on the matter. I was taught the food pyramid in grade school. It's barely relevant, and it was literally the only diet and nourishment education I recieved from my first world primary/highschool education. Unless you are going into health science or nutritionist type college credits, nobody takes the time to learn anything further about it later on. They just eat, and don't really think about it. I certainly didn't for a very long time.

Additionally, when I learned about the food pyramid, the examples didn't really make a lot of sense to me, since at the time I had barely touched any food preparation tasks, nor dealt with food that wasn't ready to eat already (usually prepared by my parents), and I had no context for what a "grain" really was, or why bread was considered a "grain" in the pyramid. I was stupid. In many ways, I still am. Yet, later in life, I don't know of anyone who is running their meal plan through a professional nutritionist before making the food. I don't know of anyone who, even if they have a meal plan, even knows a nutritionist who can consult on whether the good that they eat will provide the nutrition that they actually need.

The general population seems to put most of their trust in food makers, the corporations that make ready to eat food, to have accounted for their nutritional needs. Places like fast food restaurants, normal restaurants and those that make recipes, and most of their interest is in making food you'll enjoy, more than food that will actually provide the nutrition you need.

On top of that, even most doctors won't, by default, order tests to ensure all of your nutritional needs are met, and unless you have a symptom resulting in a significant deficiency of something, you would never know if you're behind or not getting enough of something. I can hear the comments now, "but if they're not being affected, why does it matter?" .... The thing is, they are being affected, just not significantly enough for them to be able to draw a correlation or even really complain about it.

So at the end of the day, we're probably all malnourished in some way, or at least, there's a nontrivial amount of people who are unaware that they're malnourished, which isn't being caught, and nobody has the knowledge or understanding to know it's even happening. The education on nourishment is so lackluster that is easily forgotten by most and instead we learn about factorials and trigonometry which most people never use past highschool.

I'm summary, more people than you would expect are likely unaware that they're malnourished, and the education system would rather teach you maths you'll never use than ensure you can feed yourself properly. The whole thing is fucked, and it's ironic when people lecture or question anyone about their nourishment needs, given how little any average person has been taught about proper nourishment. Everything is fucked and everybody sucks.