Because most branded items are at least 30% cheaper!
Take my Folli Follie Chryssalis watch in plated Rose Gold that I bought today:
The Folli Follie US website has it listed for $180USD in the Silver plated. Even including tax, that would only work out to $200USD. I paid $303USD in Hong Kong for it.
It is not just this one instance but pretty much everything is more expensive in Hong Kong. If you are visiting Hong Kong in hopes of good deals in shopping, I would advise you to visit Bangkok instead.
You might wonder why I knowing overpaid for the watch. The answer is that shopping for a watch in Hong Kong is not easy. You basically have two choices, cheap or horrifically expensive. Nothing caught my eye except a blinged out $130,000USD Rolex. There is no way I would spend that kind of money, even if I could afford it.
The Folli Follie watch was the only watch that I liked enough to buy and overpay for in the past 6 months of actively searching for a watch. Additionally, there is something to be said about instant gratification. I saw it today, I liked it, I bought it, and now I am enjoying it.