The idea that all vegetarians and vegans are outspoken and in your face about it is obviously absurd. After all, if they weren’t, and they never mentioned their diet to you, how would you know they were vegetarian or vegan? It’s the toupee fallacy. You’ve probably met hundreds of vegetarians or vegans and you don’t even know it, so they don’t count for your representativeness heuristic, but the ones who are outspoken about it get noticed and do fit it. So you conclude that all vegetarians and vegans are in your face and preachy about it.
The vast majority of the reason people attack vegetarians and vegans is simple: they know that the vegetarian or vegan is doing something better for the world than they are, and that makes them uncomfortable. They know on some level what they’re doing is problematic, but they’re in denial about it, and other people who are actually living up to those ideals remind them of their personal moral failure. So they attack them to discredit them in some way.
I’m not going to debate why meat eating is a moral failing. It’s obvious. You’re causing suffering because you like meat. Sure, you’re going to come up with some bullshit hypothetical that has absolutely nothing to do with anything you actually do (but what if an animal lived a perfect life and died painlessly!), the reality of factory farmed meat is torture. We are condemning billions of animals a year to a life of nothing but suffering so we can have a $3 chicken sandwich. Watch a documentary on a CAFO. We would be mortified if someone was torturing our pets, so we understand that animal suffering is bad and we can even empathize with it when we choose to. Those animals absolutely suffer and we do it for our own pleasure. But acknowledging that is hard to swallow. So most people are in denial about it, avoid thinking about it, and attack anyone who makes us think about it.
So when we see someone making the choice that we should be making, it makes us feel bad about ourselves. We get cognitive dissonance. We attempt to resolve this dissonance by discrediting the other person - calling them a hypocrite or saying they’re just getting a thrill for being high and mighty and criticizing meat eaters, whatever. Something that takes them down a moral peg or two so they’re no longer superior to us.
You see the same sort of attacks when, for example, someone tries to live their lives in a more environmentally friendly way. Then suddenly everyone is watching them like a hawk for any sort of tiny wasteful thing they do (like Greta Thurnberg eating something that came in a wrapper). We don’t like that they’re actually doing something that we should all be doing and we aren’t, so we look for reasons to take them down a peg to alleviate our own negative self-evaluation.
And yes, I know there are non-ethical reasons for being a vegetarian/vegan, but you’ll notice that for example if someone is vegetarian for religious reasons they are far less likely to be attacked than someone who is for moral reasons.
And no, I’m not a preachy vegetarian/vegan. I eat more meat than the average person. It’s a moral failing. I’m not in denial about it. And because of that, I would never attack someone for doing the thing that I should be doing.