People like to characterize those they already want to disagree with by the worst, most extreme examples of the group. So before even considering the benefits/drawbacks of veganism, people have already chosen their position after thinking “vegans are just so preachy and annoying”.
It’s putting outrage before reason, and it’s really common in social media and news:
Think about how Fox News viewers picture liberals as the least coherent, unreasonable individuals that they see get interviewed, when in reality most are just normal people.
Or the reverse: how people in liberal circles might see conservatives depicted as Maga-hat wearing weirdos who think 5G is killing them.
Same with how many religious people evaluate atheism or atheists think of religion by their worst representatives.
In short, I think the answer is that it’s a symptom of tribalism and identity-politics.