The problem with the big mac index is that it assumes people still like mcdonalds.
I assume that Macdonald's prices based on perceived value. If you go to Asia, McD’s has a much higher perceived value - and McD’s knows this and does multiple product launches every year, often catering to local tastes.
So I’d assume that consumers in the US or Europe don’t value a Big Mac the same was they do in Asia and as such the pricing differ (i.e. they can price higher in Asia).