What makes you think cash is anonymous? In China, ATMs record the serial numbers of the bills they dispense.[1] 80% of ATMs in Beijing, Shanghai, Nanjing and Guangzhou had this feature in 2013, and it was supposed to be rolled out nationwide by the end of 2015. There's no secret about this; you can ask the ATM to print the serial numbers of the bills it dispenses on your receipt.
Now being rolled out in China: ATMs with face recognition.[2]
Because not all cash transactions go through ATMs. When friends are splitting bills, or lending each other money, or two people are just selling a piece of furniture, the trail gets lost. In the US that's enough to establish reasonable doubt.
It is utterly trivial to launder cash to avoid this kind of tracking. Pay for a $1 pack of gum with a $20, and now you have $19 in clean cash. Rinse and repeat as necessary.
Now being rolled out in China: ATMs with face recognition.[2]
[1] http://hkmb.hktdc.com/en/1X09V6YK/hktdc-research/ATMs-to-pro... [2] http://en.yibada.com/articles/35951/20150601/china-worlds-fi...