Why are animal rights activists against the use of fur and silk?

Because it's an unnecessary waste of life, as well as a potentially painful process for the animal. You don't need fur-synthetic furs work just as well, and to skin an animal for something so incredibly unnecessary is wrong. Plus, before the animals are skinned they are often put into tiny cages, outside, in rainy and/or frigid weather, and are generally abused. if you want to see it for yourself just google things like "fur farms abuse" or go to the PETA website for more information.