If I commit a crime and the evidence is in a Hilton hotel room in the UK nobody expects Hilton
Everybody however expects U.S. police to go the U.K. police and courts who will check if it was a crime in the U.K. in the first place then ask the U.K. Hilton to hand over the data, aka due process .
If tracking cash transactions were as easy as you make them out to be, every illegal laundering from drugs to arms would have been long shut down.
a unique serial number is not personally identifiable by itself. Cash is far more anonymous than bitcoin, yes people can theoretically track it, with bitcoin EVERY transaction is by definition recorded. With cash some transactions maybe recorded, retailers do not necessarily record the serial number of every bill of each transaction, while that is certainly possible it is still not common place. Even if all were tracked in the U.S. there are still plenty of holes in your graph, making it difficult to every get anywhere near full picture, primarily due to the nature of U.S. currency. Plenty of people in the world use $ as their medium of exchange, these people who feds have very little control over and can do little to track either.
This is BS, Crypto-currency has very little to do transaction privacy or central control.
if you are worried about privacy of transaction then use cash!
If you want use anything that has no value on its own as money be it paper (cash), plastic, or Hashes as a store of value, then you will have to trust someone to enforce it. Otherwise you are going to have volatility, and people losing money in scams just look at the ones happening in bitcoin.
I am broke / I have only this much cash / I cannot afford it despite your "attractive" financing / My wife/girlfriend/partner won't allow me to..
Life is cheap, but the accessories can kill you.