It would seem that this makes it significantly easier to stay under the radar -- simply set up your credit cards to suggest a very normal life and then use cash for any illicit activities.
And, if a group of people (such as a terrorist cell) wanted to, it would be easy to deliberately create a paper trail to mislead authorities.
You get promoted based on how many cases you solve, not how many difficult cases you solve. If you are smarter than the average criminal, you probably stand a less-than-average chance of being caught.
because the authorities have no control over the supply of cash, would never try to outlaw cash, or never require banks to report the withdrawal or deposit of cash.
And, if a group of people (such as a terrorist cell) wanted to, it would be easy to deliberately create a paper trail to mislead authorities.