As long as non-vegan leftists aren't anti-vegan, I really am not too bothered by their existence, at least relatively speaking. I find the fact that anyone, regardless of political ideology, consumes animal products to be an unpleasant fact to acknowledge. Still, I think it ultimately all comes down to ignorance versus malice. When a non-vegan leftist just hasn't adequately considered veganism, it's much less bothersome to me than, like I said, when they're actively against veganism. Of all the viewpoints you could change people's views on, veganism might be the hardest because it requires a drastic move to a different way of living. To follow through with their newly acquired view that animal liberation is crucial comes with a hefty amount of changing their actions, and that can definitely make it hard to convince someone to change, especially if it's radically different from the lifestyle they're accustomed to (i.e. convincing a vegetarian who only consumes dairy every now and then to go vegan is much easier than convincing someone who eats ham and cheese omelets every morning for breakfast on top of meat in every other meal to go vegan).

It's also worth noting that people take the criticism of animal product consumption very personally. If a vegan says "Eating meat is ethically wrong," an omnivore who's never met that vegan in their life might feel personally offended because they don't want to feel ethically wrong as an individual. When I criticize carnism, it's a critique of how it's been engrained in so many of our brains, and it would be better to take that mentality out of our brains, as a society, as much as possible.

If carnists admit that you are right about your view that consuming animal products is wrong. In that case, they'll feel inclined to disagree because if they agreed with such a viewpoint, they'd either have to A) live with being a hypocrite (which is considered very shameful for a lot of people) or B) actively make lifestyle changes to veganism that they would find "too inconvenient" for them.