You want to keep arguably the biggest possible target for robbery open 24/7...that is just stupid. I know that wasn't your point, but do you know how many more bank robberies there would be?
While that would put people to work, it would cost the bank more money. Then, they would need to charge you more for their services (whether through fees or low ball interest rates). Additionally, the responsibility of management is to its shareholders; since so few people would bank at night, this would not be a cost effective method of customer service. As such, the shareholders would be upset and replace that management with people who would make the bank more profitable.
The bank isn't a public service, nor are they your friend. They have a business with a profit motive just like every other business. I am not saying that they are evil at all, you just need to understand their goals.