the arbitrage can exists on a regulate market. the fiat currencies are backed by a Central Bank. The forex companies MUST be licensed, they are verified, audited and so on.

Bitcoin maket is unregulated and so the most of the exchangers. Once, a company is not verified, it can do what ever it wants on an unregulate market.

You cannot compare the real forex market with the Bitcoin market where 90% is black market and shady exchangers

what real arbitrage can exist when ALL the big exchangers are using the Willy bot (same as MTgox) ? I think the chinesse exchangers have 80% less volume than the volume they show on their website. This is called price manipulation not arbitrage.

Willy bot increases the volume so that the potential clients to see how "big" is that exchanger and how many clients it has AND to manipulate the price for the exchanger interest too

in fact, the market is very low but the price manipulation remains.