The concise answer: most people have no taste. That said, although most of the stuff in the A|X shops is poorly cut and made from truly inferior materials, there is occasionally a good find: a leather jacket here and there, a reasonably priced and well-fitting t-shirt, etc... The problem with mass-marketers like A|X, Banana Republic, and French Connection, in my opinion, is that their clothes need to appeal to a broad audience that is at once conservative and anxious to be rebellious. One would think that a trend-following company like Banana Republic, for example, would have no problem bringing Gucci and Prada copies to life. In fact, they might theoretically even do one better, and work out the kinks in any given collection. However, the designers working for these companies have many constraints. Most importantly, they lack the luxury of an imaginative and daring audience that designers enjoy. The result is stores like A|X and Banana Republic, which are sort of like the Readers Digest of designer lines. You sort of get the idea, but the real power, the real "oomph", of the original is missing.
