Quick answer: because Sunny D is an imitation of orange juice and not the real thing, and the oil acts as a surfactant and keeps some of the other fake elements in suspension. Never drink this crap.

Drink Florida's Natural Orange Juice, which is the only national brand which is made from U.S. grown oranges and not from concentrate imported from places like Brazil.

Vegan Mom is incorrect that I am shilling for the above brand. It is the only juice we buy because we are informed consumers, not because we are being paid by the Florida orange growers. Get it while you can, because the U.S. orange crop is being besieged by a bacterial disease which could wipe it out. I have no trust in foreign producers' products, as they routinely spray their fruit with products which have long been banned in this country.