Parallel Processing does not work in ArcMap with 64-bit Background Geoprocessing enabled

Cause

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)

Workaround

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: 

1. Go to the Geoprocessing tab, and click Geoprocessing Options.

2. In the Geoprocessing Options dialog, uncheck Enable Background Processing.