Because outside of the initial production cost, which for skins is almost nothing and Valve doesn't even make them, but the ability to replicate them are infinite. The prices of skins are based off of the user made community, not Valve. It's a marginal supply because you can only get the skins "by chance" from keys and cases, but their supply is infinite.
It's not like a cancer drug treatment where it might only cost 2-3$ for the treatment, but if you limit the amount being sold, you can charge 100$. Tangible goods to the classic economic demand and supply schematic is not as practical in that way.
Not to say that digital goods can't have value or be scarce, they do that on purpose so that they can price accordingly. So for example even something that costs 0$ to make, would still cost you a hefty penny if they forcibly make it hard to obtain. So that's less of a supply and demand, because supply was never an existing issue to begin with, just a demand, and a chance. That's basically how information and data work, the data could have no practical value but can sell for a lot of money if you limit it.