Maybe your DB creds are in a secure configuration system and maybe the KMS credentials are granted by an IAM instance role. If only certain users are able to get access to config (ie RDS), and only certain machines are able to decrypt data, you now have a multilayered security approach where an attacker needs to breach both a user and a piece of hardware. If you breach a user and get your hands on a DB leak, you can't use it because you don't have access to the hardware. If you breach the instance role then you can make decrypt requests, but you would have also needed to have breached the data separately and independently.

Don't forget that data is most often leaked by people. DB backups, dev environments, inappropriate transfers. We hear about the big targeted attacks, but probably 90% of actual breaches are due to human misuse.