> If one company has margins that are too high, another steps in to take some of that profit, by offering at a lower price until it reaches equilibrium.

Right, but that mechanism is only possible if information is public. You don't know about the pricing of growing, harvesting, shipping and selling bananas - but it's possible to find out, and a company looking to enter the market would find out. You can rely on market pricing because the market lets you outsource your research to other market participants, but that only works if research is possible in the first place.