You don't need a gun to rob a bank.
Obviously this guy in the movie doesn't need a gun, he has a bomb, why even bother with a fake gun?
2. Banks don't give a crap about protecting their cash anymore. They're all insured. Many banks have an official policy to give over cash to anyone that demands it, no need for a weapon. You don't even have to make a threat, just say "give me your money" you can be completely empty handed. They give it to you no questions ask. I wish Hollywood would catch wise to this, because it would certainly make for comedy to have a bank robber who just walks in and out, doesn't make a scene and doesn't make any threats.