I think this is more of a new thing. I've talked about this stuff with people who are now in their seventies and eighties, and they basically seem to get that animals suffer in substantially similar ways to humans.

The difference is that humans are broadly capable of understanding the moral consequences of their actions, and in the west we seem to have developed the concept of showing mercy to animals (stunning and sticking rather than bleeding animals out through the neck while conscious).

While some animals have more complex proto-moral behaviour, they would not be able or willing to extend these same courtesies to us, in the reverse circumstance.