Because Spring is 15-year-old software, and as such was originally built for single-core processors. There have been projects here and there to retrofit multithreading into the Spring engine, or even rebuild it from the ground up to support multithreading, but those have been difficult and slow-going.
Building multithreaded software is a monumental software engineering challenge when done from the start, and retrofitting software to be multithreaded makes even that look like a walk in the park. At this point I'm pretty sure there is some threading in Spring, though I'm not sure entirely where the separation is (probably between graphics data transfer and everything else), but unless some breakthrough in structuring the physics code comes up that allows arbitrary core usage while keeping the different cores' parts of the simulation in sync with each other, I doubt there's much that can be done to improve CPU usage.