It's due to the difficulty getting the fiat out of the exchange, then out of the bank, and then out of the country, and then doing this in the amount of millions of dollars per day. If you try, you'll find your account frozen fast. It's not complicated. That's all that's happening. So instead the price pressure is balancing from petty trades from opportunistic small men. Not big investors. Not yet, at least...
Also some asian exchanges tack a 1% fee on withdrawals. That is also a limiter. You'd pretty much have to pull a Rothschild's to exploit this opportunity. Meaning, have large amount of fiat in both nation blocs and buy and sell solely on intelligence.