When transit_encryption_enabled is not set to true in the aws_elasticache_replication_group resource, data transmitted between ElastiCache nodes is not encrypted, increasing the risk of data interception or unauthorized access while data is in motion. Without encryption in transit, sensitive information can be exposed to attackers with network access, potentially leading to data breaches. Enabling transit encryption ensures all traffic between nodes is protected, safeguarding against eavesdropping and man-in-the-middle attacks.