That's a fair question, my comment was getting long in the tooth.
So, let's take a commodity like a wool blanket (or some other simple commodity to illustrative purposes). That blanket has more value than the wool that made it, but where did that come from?
When the wool becomes a blanket and gains value, it does so because someone put the labor into converting the wool to a blanket. So the generic function here is commodity + labor = higher value commodity.
This goes all the way down. The wool comes from someone putting in labor to get it from the sheep. And it continues up, the blankets become part of hotels, bedding, other higher value commodities, etc.
More expensive commodities are more expensive because they need more labor to produce.
So a commodity is essentially a crystallization of labor value. It represents a complex chain of labor value multiplying other labor value.
So back to your question. CMC transactions are about using gold to compare labor value crystalized in commodities. I'm selling my blankets to an iron miner, but I don't need iron ore. But with a universal money commodity like gold I can go buy the commodities I do need.
The MCM transaction has you buy my blankets and sell them later for more than you paid me without adding labor. If the value has increased, but no additional labor has been done to it, it's still the same blanket I made. I can also sell my blankets at the higher current price, so what have you done except taken some value from me? Where else does the value come from?