Backyard chickens is another thing, and anyone who wants fresh eggs should go that route. The same applies to chickens raised for meat.
To know that you aren’t supporting such violence, it’s easy: just don’t eat animal products.
As you mention, the problem is scale, but then everyone doesn't have to live in a city. Different solutions for different situations and all that.
Even if you treat an animal like royalty for years and decide one day to kill it for food or clothing or whatever, you are still doing something wrong. Our treatment of anything does not grant us the privilege to end its life.
We feel this way about dogs and cats, and other humans, but never pigs or cows or sheep or goats or fish, etc. And there is no reasonable explanation for why, other than speciesism.
> why do ethical vegan proponents feel it is better for an animal to never live at all?
The same reason that I feel it'd have been better for someone who abused and murdered their child to have never had any children.
Bringing something into this world does not permit us to treat it however we want. We all know that, some people just think it for some reasons doesn't apply to chickens, cows, pigs, etc.