The correct answer is: "It costs more to do business because banks charge fees for exchanges."
A problem with exchanging currency is "it costs more to do business because banks charge fees for exchanges."
Indeed that is part of the bank industry, a big part of the bank system. In every currency exchange operation, the bank charges a fee and this represents an interesting portion of their income. Because most private banks operate in free marking economies, they establish their own fees and regulations in the exchange market that is very volatile due to different conditions such as the oil prices or the stock market exchange, the stability of the dollar, the Euro, or the Yen.