How would you explain to a skeptical bank manager why the socially optimal number of bank robberies is not zero?

First we need to understand what social optimal amount means. Socially optimal amount of some good, is that quantity which is the best for the entire society(not just you own self). Therefore, even though bank robberies hurt the manager, there are some benefits of bank robberies for the overall society.

The biggest benefit of bank robberies, is that it creates jobs for a lot of members of the society. As a fear of bank robberies, banks would employ security personnel thus providing people with work. Similarly bank would purchase CCTV cameras and highly secure vaults, thus creating demand for the CCTV and vault manufacturing companies. If there were no robberies, these jobs would lost, and the security equipment companies would go under. Similarly theft insurance companies would also go bust. This would not be optimal for the entire society, and only the bank would benefit from zero robberies. To summarize, if there were very few positive number of robberies, the fear of these would be able to employ a large number of personnel and fund a variety of industries, which would improve the overall benefit attained by the society.