Because before they knew you wanted to leave they did not knew you wanted to leave. If employee A is plotting to opportunistically jump boat the company can onl

Because before they knew you wanted to leave they did not knew you wanted to leave. If employee A is plotting to opportunistically jump boat the company can only (preventively) act after it knows that A wants to leave. The same usual problems with false positives and false negatives. (also 99.9% of employees are not leaving, considering them all a risk can be expensive)