Charitably, it may be because of its superhero/supervillain setting. In real life, bank robbers seem to be commonly, or maybe even usually unarmed; they don't need to be armed because tellers always have instructions to cooperate, and being armed raises the legal penalties significantly while increasing the odds of something going wrong. (To give an example, today I was reading an IAmA with a bank robber on Reddit. He didn't carry any weapons except a hammer, in case the banks ever locked their (glass) doors while he was leaving, which one bank did try on him.)