It's about scale of pricing and production costs. It's cheaper to have someone stick a wad of aluminum into a forging/CNC machine then it is to have someone lay a bunch of piles of carbon fibre into a rediculously tiny mold.

The most expensive and lightweight aluminum stem is going to better than a carbon stem of the same price. I'd rather have a Deda Newton or FSA OS-115 bolted on to my bike than some carbon wrapped garbage that weighs more/isn't as strong/feels flimsy.

The only carbon stems that have a significant weight or stiffness advantage are custom full carbon ones made by some German dude in the autoclave he has in his basement. Custom carbon stems can be made as light as 50 grams, but they would cost over $500 (Nordischer Rahmenbau).

Again, you have to look at the top of each material's price range if you want to get to the good stuff.