It is using multiple cores. It just cannot always all of them up to 100%. Heavier codecs such as x264, x265, and vp9 will use more than the others.
The feature Steve mentioned: “Multi-core parallel image processing (when not using GPU and frame-dropping is disabled)” is for when previewing, and the “Parallel Processing” option in Export does the same thing. But, again, due to bottlenecks and deficiency, it cannot always use all CPUs 100%.