People trust by default, when there's no big flashing red warning sign.
People trust when there seems to be no alternative since no bank is competing on the finer points of security.
People trust when it looks like others trust, because there's safety in numbers.
People trust something more the longer they use it without it biting them.
All these heuristics can be wrong, but they're more practical than a theoretical analysis. Trying to reason about things going wrong is too much effort and can easily leave out a crucial factor.