Because it's the easiest way to avoid having to consider changing opinions and behaviors

People don’t like to have deeply held beliefs challenged and even less so like to be told they are a bad person for eating certain types of food.

Well it's like the old joke says,

Q: how do you know someone's a vegan?

A: OH, THEY'LL TELL YOU.

Seriously tho I don't have a problem with them unless they start saying there's no difference betwern killing an animal and torturing it.

I have no issues with veganism but I do have issues with being attacked because I'm not vegan. I've been attacked for using cow milk even after explaining that I can't use plant milk because of sensory issues.

Funnily enough, just like you said, I know vegans IRL who would never behave that way and of course I accommodate their dietary choices when I'm in charge of food.

I think it's mostly an Internet thing when you just see the loudest minority instead of the more quiet majority.

I personally don’t care if people want to be vegan and sometimes I’ll eat vegan food. For me though, on the whole, I find vegan food unappealing. It usually involves compromises and doesn’t taste that great.

Additionally, I did eat in a house of vegetarians for about 10 months once. I ate meat outside the house on occasion, but mostly ate vegetarian. It was really hard on my stomach. After meals, I would usually have crazy gas. It did improve over time, but always was an issue for me.

For these reasons, I don’t prefer veganism or vegetarianism. It’s okay if you do though.

Edit: for vegans wondering why they’re hated, I think the downvotes to this comment summarize it. Above I’ve expressed a view perfectly tolerant of veganism. I’ve even tried some of it myself, but don’t find it appealing. Rather than just accept that someone else has a different dietary preference than you, you shit on them. If you want to be tolerated, be tolerant.

It's the fandom mostly. I like using Linux but I don't think you are immoral for using windows. Rick and Morty is funny but I don't think Rick is someone to take any advice from. CrossFit seems to work for most people who stick with it but it is one of many options. I won't apologize for being an atheist but I don't think you are stupid for not being one.

The problem with Veganism is the problem with monotheism. There is one proper way to live and all the others are wrong and awful.

That and the lying. I won't deny that there are farmers who abuse their animals, that is a problem that can be dealt with through the legal system, but you can't sell me a sack of lies claiming that I abused the cows I milked growing up. Because I know I didn't.

I don't hate vegans. I even dated one for a while. She would eat veggies, I would eat meat. It worked because neither of us were dismissive of the others choice.

I do hate being preached to. I think most people do. Many of my encounters with vegans were ones where they were very hostile to me. For just not being vegan. I wasn't going around telling them veganism was bad or that they were a terrible person for being vegan. I have been told that by vegans. That I am a bad person for eating meat.

I have to eat meat I have always had a high metabolism. At various times in my life my maintenance diet had to be at least 3,000 calories. Plus of course any additional food needed for non maintenance activities. Additionally I'm allergic to soy. This means that it would be nearly impossible for me to have gotten enough food without eating meat. I am pretty certain that if I had not eaten meat and animal fat. I would have died as a child, or at best had serious issues with nutrition. I was well fed as a child. I could eat as much as I wanted, still many of my memories of childhood were ones of hunger.

I even explained this to someone once and they responded by saying "well you didn't die" and then heavily implied they wish I did. As if me not dying made my point invalid.

This is why I am skeptical of vegans and vegan ideology. It is often toxic, and actively tries to hurt and shame people. Even people like me who are effectively choiceless in regards to eating meat.

Often even non-militant vegans will allow this behavior. I know how frustrating it can be to be told you need to police your own community. Though that doesn't mean it's okay to allow bad behavior amongst your peers.

No one likes being criticized and labeled a monster by an ignorant prick. They way too frequently act like every egg comes from a half zombie chicken that's kept in a little box and tortured just for fun, or that a cow couldn't possibly end up in a cheeseburger after living its best life. Factory farming is bad for lots of reasons, but it's not characteristic of the entire industry.