A couple of major reasons. There are so many aspects of a startup that go beyond where you are operating and the kind of market you are targeting. YC's advice with these matters is very valuable. They have started accepting international companies as well recently, which is a good sign.
Finally, the value of the YC network including its very strong alumni can never be quantified.
Finally, the value of the YC network including its very strong alumni can never be quantified.