Why the "Defects" field into Test Case Execution view, it is greyed out & not searchable even though at least one defect exists?

The behavior is the same since Defects Column is available in TCER view hence I think this is how it is designed(not all columns are sortable or filterable).
There are a couple of TCER related RFEs in this workitem:
https://jazz.net/jazz02/web/projects/Rational%20Quality%20Manager#action=com.ibm.team.workitem.viewWorkItem&id=42552

but make defects column searchable is not one of them, you may want to raise one in jazz.net.