Wouldn't arbitrage require a rebalancing of the funds at some point.

If I were to simultaneously purchase BTC on one market and then USD on the other for a profit overall, then I wouldn't be able to take advantage of the situation again until the markets fluctuate to the point where the exact opposite position provides a favorable outcome.

I apologize for my lack of understanding. I'm truly trying to wrap my head around what happens after the initial simultaneous sales have completed.