One of the reasons is that for many centuries the Western thought was deeply influenced by Christian thought as formed mainly by Thomas Aquinas. According to him, animals are devoid of "the life of reason" with all consequences (basically, we can do what we want with them). It turned out, animals can feel, can be happy and unhappy, and can understand much more than we had imagined. This not a result of our intuition, but years of research. Nevertheless, the harm has been done, and animals have been cruelly abused because of the underlying belief that they don't feel (or, even if they do, it is ethically neutral). It's 2021 and there are several companies still testing their products on cosmetics, for example. At least natural fur is not a thing anymore.
It’s worth noting that Aquinas was using ‘reason’ in a quite narrow and specific sense (very roughly, the ability to think about things as such via concepts). His position was not inconsistent with dogs having emotions or various forms of what we’d call intelligence.
On the other hand, in medieval and early modern Europe non-wild animals that did bad things were often given full trials with a judge, jury, prosecutor, defense [1]. That seems to suggest a view that animals were more than just unreasoning beasts.
it also doesnt help that on page 1 of the most popular religion it basically says that humans have dominion over all the animals on earth, or some sort of slosh to that effect
