Because the time cost for finding just the right style and the right fit and the right brand that combines both those two things for you that are in the right size costs about the same as just paying full retail, depending on how much you enjoy the shopping process.

I tend to know exactly what I want, and don't have the inclination to scour the discount racks in case they have exactly what I want. And if it's not exactly what I want, I usually end up not wearing it,, which is an even bigger waste.