An analyst received an alert on their desktop computer showing that an attack was successful...

 The discrepancy described suggests that the system had a Host Intrusion Detection System (HIDS) installed. HIDS are designed to monitor and analyze the internals of a computing system for signs of intrusion and policy violations. While they can detect unauthorized activities, they do not take direct action to stop an attack; this is typically the role of an intrusion prevention system. Therefore, the alert was generated, but no mitigation action was taken because the HIDS does not have the capability to intervene.

References := The Understanding Cisco Cybersecurity Operations Fundamentals (CBROPS) course material covers the functions and limitations of various security systems, including HIDS, and their role within a Security Operations Center (SOC)1.