And what if that server needs to be rebooted some day? What if there's a hardware failure and it has to be powered off?
Something as big as PSN has multiple servers reading the same DB and must be able to tolerate failures without forcing everyone to re-enter their CC #. The keys must be stored persistently somewhere.
What we do where I work is take the newly generated key whenever we key or rekey the system, split it into multiple pieces using Shamir's secret sharing algorithm, and those pieces are distributed to several people.
Whenever the server needs to be started, two of those people must enter their key shares. That enables the server to reconstruct the key, which is then stored in memory.
Something as big as PSN has multiple servers reading the same DB and must be able to tolerate failures without forcing everyone to re-enter their CC #. The keys must be stored persistently somewhere.