If rig rental rates weren't higher than could be obtained by straight mining, then there wouldn't be any incentive to make them available for rent. There's risk on both sides of the equation, and the market seems to be doing a good job of sorting out a reasonable rent. If rates are too high then the rigs won't be rented. If they're too low then there's no incentive to rent them out. Somewhere in the middle is the equilibrium of supply and demand.

You also have to keep in mind that a lot of the rented rigs aren't being used to mine whatever's top of the list at CoinWarz or wherever. If someone wants to accumulate a stash of a fairly recently launched coin, and it's not on any exchanges yet, then the only way to accomplish it is to mine them. Plenty of people who don't have the time / space / funds / electrical capacity to run expensive rigs 24/7, so this marketplace lets them decide if they want to speculate on leasing someone else's equipment for a while.

Essentially the going rate will end up being normal mining returns + speculative premium. That speculative premium is what supply and demand is sorting out as we speak.