This is a known limitation/as designed behavior, and has been reported as the following defect:
NIM087308 Parallel Processing does not work with 64-bit Background Geoprocessing at ArcMap (ArcGIS for Desktop)
Parallel processing is not supported in 64-bit Background Geoprocessing. Users can either run tools that support parallel processing with 32-bit Background Processing set up in ArcMap or migrate to ArcGIS Pro.
To make sure that parallel processing works in ArcMap (ArcGIS Desktop), follow these steps: