If imports are higher than the exports for a nation the value of the currency is negatively impacted which is one of the reasons why governments want to know how much money goes in and out the country, rightly or wrongly. It is an economic indicator.
Bitcoin being transferred overseas would not necessarily trigger an export nor an import from the country the bitcoin is being transferred from. It would be very well possible that someone from country A send money to country B to have a product produced in country C shipped to country D.
Additionally imports and exports are measured by the value of goods into and out of a country, not by the value of currency that flows into and out of the country.
The only reason a country would need to monitor money flows is to monitor the value of their currency, however this is not applicable with bitcoin as these transfers would not affect the local fiat currency.
It's the amount of currency in basically every country minus a few.
Think about it - some countries only have one export (Oil, tourism, etc). Do they measure value by goods? Do they measure value by amount of currency?