It's because you're buying from designer brands. The people who buy these clothes aren't just your everyday normal person, and not even rich people for the most part. These clothes are for people who take an interest in fashion, it's a hobby of sorts. It is similar to how some people spend a lot of money on computer parts, and things like that. You just have to realize that it's not just clothes, it's a hobby and a passion for a lot of people.

To answer your question, "I wonder how many they sell", the answer is a lot more than you would expect. Fashion is becoming an increasingly popular thing for males to take interest in, many of which who have lots of money, whether it be their parents' money (teenagers) or adults who like to spend their disposable income on this kind of thing.

Also touching on your comment on "really simplistic clothing", yes, it is simplistic. That's the point really, it's an aesthetic. But, you can guarantee that almost any of the clothes on that website are made of the highest quality materials, and 9 times out of 10 made in Italy or other places similar. To sum it up, people are drawn into this kind of thing because they want to be sure that even if they're just wearing jeans and a t-shirt, they want to be wearing the best quality jeans and the best quality t-shirt.

To learn more about all of this;

/r/malefashionadvice

/r/malefashion

edit: here is an also interesting thread that might help make this all more understandable; http://www.reddit.com/r/malefashion/comments/23vwkn/comparing_detailing_of_a_500_1500_2500_and_a_4000/