Encrypting data at rest in a data center doesn't provide as much protection as you might think. The level of system access in order to access the raw data files

Encrypting data at rest in a data center doesn't provide as much protection as you might think. The level of system access in order to access the raw data files is usually about the same as accessing the higher level database system itself. So at best you're adding a couple of extra steps to the attack. Since there are usually significant costs to encrypting production databases, such as CPU utilization, many organizations choose to focus on other security controls. Data encryption is just one of many security controls at play in a scenario such as this and the security team has to carefully pick their battles to avoid just being completely ignored as the "say no to everything" guys.