I'm getting the sense they have a problem acknowledging their own mortality. Denying eating flesh allows them to hide from the idea they will be eaten as flesh one day. My experience is that vegans and vegetarians will shut you down hard if you try to discuss anything relating to death, crime, violence, rape, or bigger picture concepts like religion or science. You get the sense they need to be validated in their ideas that they are in control of their own lives. I think it's a manifestation of the anxieties associated with anorexia, whereas anorexics try to take control of what goes into their bodies by taking in nothing, primarily beginning with teenage girls and obviously being a deferment of their anxiety about being penetrated, vegans refuse to take in animal products for the same parallels, their anxiety about being eaten/dying or not having control over their bodies.
One I knew very intimately became vegetarian because she wanted her parent's to stop taking her to McDonald's because she was afraid of getting fat. Nothing about animals or environment.