Gold mining companies compared to gold

Most large mining (and oil production) companies hedge their exposure to the price of the actual commodity using financial instruments like swaps and options. Yes it would be great if the price of gold doubles and their revenues doubled, but it would be catastrophic if gold tanks, and many miners aren't willing to take that risk. When you invest in a commodity producing company, you're investing in the operations of that company as much as (or more than) the value of the commodity.

That's not to say that companies just blindly mine despite the price of gold. If the price of gold drops, companies can ramp down activity due to less revenue, and ramp back up when gold prices go up. That can cause a lag in the financials of a company relative to commodity prices.

Secondly, stock price is not always a reflection of current profitability. Stock prices reflect all expected future earnings, so investors might wait for the price of the commodity to go up more before investing in producers. You can see that the index prices have risen slightly faster than the commodity index as the commodity has remained higher, signifying the market's expectation that prices will remain higher longer.

Also, these commodities are a finite resource, so while mining technology might improve, it gets increasingly hard to find and extract the minerals, so cost can certainly rise over time.