Because there is no fixed exchange rate.
Go to your bank, ask how much it costs to buy US dollars. Ask how much it costs to sell US dollars.
Go to your credit union. Ask the same questions.
Go to a foreign exchange shop, unconnected to a bank. Ask the same questions.
You now have six different exchange rates.
Do the same thing tomorrow. You now have twelve different exchange rates.
The rates you see quoted in the paper and online is the one used by banks when negotiating with other banks. Deals in the millions of dollars/ yen/ euros/ rand/ dinars. It can be useful but it is not what you will get. And those rates change not just daily but minute by minute.
In addition all those institutions you asked will be collecting a service fee for exchanging your money.
And so does Paypal.
