Religion

With appropriate apologies for the limitations of social science blah, blah

https://www.psychologytoday.com/us/blog/animal-emotions/201207/brain-scans-show-vegetarians-more-empathic-omnivores

If this is correct, it's not so much a well thought out philosophy but a morality based more strongly on empathy than other factors. (Disclaimer: I believe most people base their morality on feelings and intuitions, even if they claim they don't. The feelings are first, then people rationalize their feelings with moral philosophy). I can certainly come up with a moral philosophy fit for a vegan and defend it too, but I won't bother to do that here.

I haven't asked any vegans what they think about the incidental suffering you mention, so I don't know what they say. Mrs. Blue is a wannabe vegan who doesn't feel well without meat in her diet. She's definitely all about empathy and not at all about moral judgement. She does sometimes feel guilty about eating meat, but I tell her she has to take care of her own health. I'm not going to ask her about rodents eating cement because that would definitely freak her out. It does bug me, so I'd be in favor of maybe some less cruel poison methods. Some animal rights groups push that sort of thing and a lot of them are vegans or vegetarians, so they are not totally inconsistent. And some, I think, are consistent to a fault - they'd rather let the mice run around with all the problems that can cause than kill them at all.

As for feelings of moral superiority, I see that all over the place, especially on political boards like DU, where we talk about those cruel conservatives all the time. It's moral hazard, so to speak, of being passionate about moral issues. Maybe most of the vegans you've met are the outspoken ones that get in your face. Most vegans I've met don't push it on anyone who doesn't want to hear it.