I'm amazed to hear that the major ammo manufacturers use "the cheapest junk they can get their hands on." Sure would like to see an authoritative source for that little gem.
Nature of the beast. Commercial Firearms reloading powders, or loading powders, are a VERY small part of the powder makers business. Think about it. How many cans of powder do you have to sell to make up six duffle bag sized bags of cordite, used in the Missiouri's rifle, firing one shot?
Each old 105MM cannon shell holds more powder then a lot of people use in a year.
Besides, what's the most important factor for a government contract? It's not the recoil from all those 308 rounds, it's does the bullet go as fast as it's supposed to, and, does it go bang? But, the number one question is, is the powder cheap enough so we can bid low, get the contract, and make money on it?
This carries over to the big companies, who, are also in the same business. The powder makers make a huge lot of powder for a military contract, and sell the over run to the commercial ammo makers, who then buy it, sell the same powder at 10 times what they paid for it, in loaded ammo. So, we bought all this powder that works best in large rifle cases. If we use that same powder in large pistol cases, we don't have to buy another powder just for some little market, like 44 special.
That's the advantage Corbon etc. have, is they can buy small non-commercial powder batches, and, optimize that match of powder to caliber, for intended use.