I agree it is concerning that there is no bank security guard in high crime areas. A lot of banks already make enough money off of bank fees, credit card late fees, and interest payments from car and home loans, so they can afford a security guard to protect the bank's costumers, machines like ATMs, desktop computers, laptops, printers, and the staff like bank tellers, and customer service staff.
I probably use a bank in a better neighborhood, or near-by town if the bank's ATM does not have any security guards. Some robbers who are drug addicts, and drunks can become very violent, and may even have weapons like a knife which they also use for self-defense if they are homeless.
Nobody gets mugged at ATM's here, you have a much rougher time at a convenience store or Metro PCS.
Sometimes, victims of muggings don't report the crime because they lost under $100. Some people like manly men may feel ashamed that a robber stole their money, and feel they are less manly because they can't defend themselves. These manly victims who want to appear manly maybe less likely to report the crime to the police which has a lot of manly police men working at the Police station.
A security guard is useful for preventing non-theft related crimes like stalking, harassment, voyeurism, and vandalism which can happen near the bank's ATM.