Well why shouldn't it actually use all our PCs resources?
Its why I use a high end PC instead of an Xbox.
Using every core isn't necessarily a good thing. Multithreading isn't so simple as "Make as many threads as possible so you can get usage of every core."
Sometimes there just isn't a good reason to make a thread. Each thread will add overhead to the game.(One core has to oversee all this stuff after all)
If there's no good reason to thread a process, you're adding more strain on the processor for no reason at all.(And even then, it still won't necessarily use all the cores)