The arbitrage is on the labor. Find someone who can build x for y and then sell it for z. The arbitrage opportunity comes from creating or obtaining a market you control. That market can be everything from a platform(Apple) to a collection of sales channels(Oracle) to a monopoly IP position(Microsoft). You do not need find people because they come to you because you control the market.

> In that case, why not trade any random commodity?

Its much easier to gain control of markets that are new as opposed to established ones.