Small amounts add up and are easily written off. Couple thousand here and there and it bypasses any oversight.
It's not the banks as an institution but the people who work in the banks.
In other words when you tell them you're a victim of fraud they can clawback the money. But instead of giving it back they can say "sorry, we were unable to get the money back" and just pocket it instead.
And yes people can be fooled, that's why scams work so well