Not utilizing all cores fully means that the CPU is more powerful than the game needs, not that the game is unable to exploit many cores. For example, at 3 GHz

Not utilizing all cores fully means that the CPU is more powerful than the game needs, not that the game is unable to exploit many cores. For example, at 3 GHz and 30 FPS, there are 100 million cycles per frame. If the game requires 250 million cycles per frame spread between a few threads, it will never need more than two cores and a half in aggregate, plus waiting; and with 10 cores used 10% there's more time to wait without dropping frames than with 3 cores over 80% load.