> I simply cannot comprehend the callousness of people towards animals back then.

Just go to rural areas, where livestock farming is a common activity and people are confronted with animals as part of their daily life and you will see that animals are not always treated kindly.

For example, they may treat live chicken like you treat a KFC menu. It is food. When they want to eat chicken, they pick one up in the coop and kill it with the same detachment as you picking up your menu at the counter. Which is ironic because that farmer is probably treating his chicken way better than the ones destined for KFC.

Wild animals are just pests, if they cause damage to the crops and livestock, or game if edible. Dogs, horses, etc... are for work, guarding, herding, etc... they get some love, but they are nothing like pets.

In that context, animals are indeed more like "things". They still get some consideration, but so are things. Most people care for their cars for instance, they want it stay in good shape for as long as possible, same thing for animals.

I think the reason we care more about animals now is because as city dwellers, we don't really live with them, we have pets, we go to zoos or watch cute videos, so we have some unnatural empathy for them. Until they cause problems that is. Empathy for rats tend to go down rather quickly for people with a rat infestation.