Building a sustainable arbitrage business requires scale. Margins are thin and should always be getting thinner, so to remain viable, you have to pursue ever more exotic arbitrage opportunities. Arbitrage opportunities encourage a speed race, and the way to win that race is to use ever-increasing resources to elbow out competition and discourage newcomers.
Usually a currency arbitrage opportunity provides a fraction of a percent in gains, like .1% max. So, you need massive amount of capital to actually make a reasonable profit. You would think why risk huge amount of money for small profit, but because the markets are established the risk is nil. As traders act on the open opportunities markets become more efficient, eg the spread shrinks, and the opportunity quickly dissapears.