Most of us are probably self taught and picking up scraps of knowledge here and there- so it's time, money, and training constraints. Most of us would probably love to do all of our own stuff, but aren't quite at that level yet, or don't have the means. Personally, when I buy character packs, I do it for the texture maps, and then apply them to a figure I spent time setting up. I rarely ever use the actual character. As for clothes, you can buy them and then create your own textures. I'll also buy things so I can study other techniques- which brings up another point. You get better variety when you decide to use different models, and not just your own, because you can combine your own ideas with someone elses. It broadens the mind and pushes you to be more creative, because you are exposed to more. Why do you think art students have to take classes like art history?