Paper money is tracked all the time. US bills have serial numbers. This has resulted in the arrest of bank robbers, ransomers, and hijackers in the past.

For money to be effectively tracked it needs to be scanned and recorded at every transaction the reality is that most bills especially lower denominations tend to circulate without being deposited, in fact many of them would circulate until they are in such bad shape that they have to be destroyed without ever being deposited back at a bank.

Even when they are deposited at a bank the banks don’t tend to keep those records forever, they have lists of serial numbers that they need to watch for but if you deposit cash into your account the bank would hold the bill information for a relatively short period of time and they don’t actively share that information with LEOs or other agencies unless explicitly requested to do so.

Even with robberies it sometimes takes years or even decades before enough bills surface to actually trigger an alert and many of them never do.

Not to mention that there are billions in counterfeit dollars in circulation at any point in time and many of them are rarely caught because they never reach a point where a good enough forgery would be discovered.