It's similar to buying a Versace t-shirt for $400. It's not that the t-shirt is the highest quality most comfortable t-shirt you've ever worn, it's that people will notice it's Versace and respect you for it. You might be able to get a 'higher quality' t-shirt from wal-mart, but it just doesn't have that same effect.
People /do/ buy knock-offs. The thing is, no self-respecting person that likes fashion will feel comfortable wearing a knockoff. It's like lying about yourself, it feels wrong.