Porcupine73 is closest to the correct answer.
Consider the materials used in an AT. Most components, including the bearings, are made of steel and of course you have the band or clutch plate materials made of cellulose with binders, or made of composite materials; and then there is the case, made of aluminum. There may be a few sleeve bearings of brass or bronze for straight shafts and such (such as the gear selector shaft), but the majority materials are either aluminum or steel.
Now the cooler is usually a copper tank inside the radiator and many external coolers are made of either copper or aluminum as well.
Aluminum cases and coolers are an alloy of aluminum, copper, silicon, and a few other elements.
So one must have a soft metal deactivator to reduce interaction with the copper coolers and the aluminum case.
In addition, any metal that goes into solution in the oil will cause an acceleration in oxidation.
Hence, metal deactivators and anti-oxidants play an important role in the makeup of any AT fluid.
As decribed in the techincal papers in
"Science and Technology of Lubricating Oils and Additives," ATF fluids have Anti-Wear additives but do not contain EP additives.