There is this mindset “If I can’t see it it doesn’t exist”. I saw a discussion on lemmy.world where an anti-vegan claimed that animals don’t have emotions since they can’t speak, so it’s ok to kill them if it can be done painlessl. They stopped arguing that point when I pointed out that by his logic I could eat my newborn.

Not too long ago the mentality was that babies can’t feel pain and so they underwent surgery without anesthetic, in general there is a lot of horrible child abuse that was seen as normal child rearing since the baby/child couldn’t articulate themselves in a way they were used to.