I do see what you mean, eric, thought it may simply not be reasonable for limited production parts to be cold forged, which takes a massive machine and stamps which are good for THAT part only. A cold forged part is a VERY high effort, resource intensive part and you need to make and sell many parts to justify the cost. And if the part doesn't come out right, there's no easy way to go back and retool. All "billet" parts are all made from the same CNC (computer numeric control) machine and are "programmed." VERY easy to make a one-of-a-kind-part.... if you can think it, you can cut one out! If the part doesn't work, reprogram the machine and zap out another one! If the parts are to be used under stress, THEN I think they go the extra mile and use a heat treatment process.
I remember when the cold war drew to a close and many ex-aerospace CNC guys got jobs making high-tech bike parts... some incredible parts came out of the process, very pretty... fork crowns, cranks, brakes, etc. It didn't take long, however, until people began to realize that the billet parts, as pretty as they were, were no match for the cold forged stuff. Fact is, cold forging can ONLY be practical if you're a large shop and you're turning out a LOT of the same parts. Some of the lightest and most durable parts are still the boring ones (cold forged) made by the likes of Shimano and Cannondale.... This fact is very frustrating to many cyclists, many of whom have the money to pay for high quality, but who'd rather not give their money to the big guy on the block. Think Microsoft.. many people are frustrated with the "monster" but it's still the best game going.
Lets face it, there's a zillion different types of engines and components, and lots of little guys with limited resources are zipping out parts using the CNC process (because that's their only economically reasonable method for doing it.) I'm guessing they make up for using billet by overbuilding the part and heat treating in some way. Seen the Orange County Chopper guys lately? They bought a cheeseball CNC machine and they're knocking out CNC parts too! Provided the parts are well designed and there is enough material, the parts will hold up.... but billet is a bit like milling something out of a bar of soap... the material is in its softest state, it's EASY, and that's why its done.
Having said all that, Al is right, I don't know if you could cold forge a piston. Just a bad shape. I think you'd HAVE to either pour metal into a mold (drop forge) or CNC it. But why go to the trouble of making a big old complicated mold for a few racing pistons? Just get poindexter over there to program the CNC machine! My guess is that the most important difference in the resulting piston from each of the respective processes is the quality (and suitability) of the alloy used and the quality of the heat treament process.