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.