Vegans who preach are like anyone else who preaches: they believe their message (usually) and are incredulous when they meet someone who doesn't. "It convinced me, I'm clever, so it must convince you or otherwise you are stupid".
Vegan propaganda, when it's not manifestly untrue, starts from the premise that people don't know that the little lambs in the field are future lamb chops, and if only we knew that - a fact that, presumably, vegans didn't realise until their epiphany - we'd agree with them.
Nope, I'm well aware that meat is dead animal, and I know how it's produced. Within limits of responsible husbandry, that doesn't bother me. The "eww, but I bet you don't eat..." part doesn't work, because offal's great and I'll eat pretty much anything and my objections are just "I don't like" rather than "ewww" (chicken feet are boring, chicken gizzards are great).
The "rotting flesh" part doesn't work because I have on occasions plucked, drawn and hung my own game. I even once did the "hang a brace of pheasants by the neck, when one of them rots through and drops to the floor eat the other one" thing, although it was I admit quite high. And the only reason I haven't shot game is because the small amount of clay pigeon shooting I've done tells me (a) I'm crap and it and (b) my shoulder hurts.
On the other hand, I could happily live on South Indian vegetarian food and (pro tip!) asking for the "south indian veggie" meal on planes out of London is the best way to get a decent meal in economy.