Earlier quoted context omitted.
Better security is not letting them get the cash at all.
Cost of dealing with a dead teller is probably higher than the amount of cash that will satisfy most traditional robbers. If that robber-satisfying amount can be recovered with a certain degree of reliability, the security model is effective in deterring attacks, minimizing attack damage, and ensuring physical safety of team members.
There are only around 5,000 bank robberies in the US each year. If they each walked away with 100,000$* which is unlikely that's only 0.5 billion which is peanuts vs the 44 billion retail loses from shoplifters and other issues.
PS: 100k in 20's is ~11 pounds. Some people might leave the bank with more than that, but not that much as most branches don't much have cash and simply simply moving it becomes difficult.