Cash is material. It may be stolen so there's a security risk in them storing it, transporting it etc. There are also the costs of storage and transportation.
Even though it's also possible to steal electronically, it takes a far higher skill level and it's somewhat easier to track.
Whatever.. The banks are the worst lot. They rip a new ring for us with their insane charges and then insult us by telling us they care so much about us.
My business would save thousands every month if we didn't get ripped by the banks and Telkom.