Argentina has different exchange rates for locals and foreigners. There is also a black market rate. Locals who have access to the black market want tourists to pay in dollars as it is the best deal.

If Argentina actually had honest exchange rates they would have about the right reserves. However they are using a false rate for most and so switching will force them to use the real rates which would devalue specific things. (They have export restrictions on many products because they can't control the exchange rate)