Many scams depend on the victims making a cash withdrawal from their bank and then converting the cash to a different medium such as gift cards, Western Union transfer, etc. As far as the victim’s bank is concerned, it’s their customer making a cash withdrawal, thus no paper trail exists.
In those cases where the scammer is able to move funds directly from the victim’s account to the scammer’s account, the scammer will quickly move that money through multiple banks, often offshore or withdraw it as cash. Even if the banks can trace it there won’t be anything left to recover.
There are also banking security and privacy laws at play, requiring law enforcement to serve a search warrant on each bank in the chain which gives the scammer quite a head start.
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