Leather is a by-product of dairy and beef production, there is vastly more leather than we use for garments. Most of it gets processed into pet food or makeup or automotive lubricants or who knows what
Not everywhere. Many places its much more sustainable to make clothes from the animals you are eating and it makes sure that you aren't wasting any of the life you've taken that you need to survive.
Wool is one of those natural fibers that can be harvested without harming the animal. Even if you end up eating the goat/sheep, it can provide a few coats of wool before hand.
But one could also use linen, hemp, ramie/urtica/nettle. However, they are more complicated to process and as the results are textiles, they are not windproof or water repellent.
Organic and recycled cotton is a lot better, and hemp and linen are also pretty good. And if you're worried about hazardous pesticides the majority is used while growing feed for animals.
