Answering in this context: because Argentina had a good statistics institute that was destroyed by three governments. So now you are paying people for lying in your face.
And speaking generically not all countries have a private sector that can afford country wide statistics research.
It was not destroyed. Some clever guy realized that people automatically trust statistics and you can fake them. Locally, INDEC lost all credibility years ago. The Economist just realized it, next time they will realize the "big mac" index is also totally useless here. Meanwhile people are having 25% raises in dollars every year, meaning that temporally we have a lot of money, until all business go broke, that is.
I don't know, the big mac index does a fairly good job at being an independent measure of inflation (and by a company that isn't trying to manipulate the price to hide inflation).
Big mac's goal is to measure currency exchange rates, which I think is a little wonky since county to country a big mac doesn't work out so well, and I wish the used it to focus more on inflation.
And speaking generically not all countries have a private sector that can afford country wide statistics research.