- The blockchain doesn't always show where the money is.
- You often can't follow the money to an identifiable organisation.
- Many organisations can often, and sometimes must, ignore requests.
at any given time you know where these 69000 bitcoins are,
It isn't that easy. Let me quote some text from my answer to
Is it true that all the bitcoins in existence are on the blockchain (the ledge), but it just depends on who the owner (address) is?
In block 669188 there is a transaction 1218…d33e. If you look at that in a blockchain explorer such as blockchair the details will be shown as
Where did the 0.00149566 BTC end up? At
1C7c…4Q1q or at
3Bwb…ECRY? No one can say, it may not be a meaningful question.
At every transaction, all the input bitcoins are completely destroyed. New bitcoins are created. There is no record of which destroyed input bitcoins correspond to which freshly-minted output bitcoins. Such a correspondence may be impossible to infer.
you just need to follow where these bitcoins go until they are being transferred to a bank or a service,
There's no obligation in the Bitcoin network for a bank or service to make their receiving bitcoin addresses identifiable.
In general bitcoin addresses are not explicitly associated with places, people or organisations. In some cases, using a lot of work studying the blockchain and making test transactions, it can be possible to infer some relationships but this is not guaranteed.
Additionally it is best practise to change the receiving address for each transaction, precisely to avoid this sort of tracing.
and then you can ask them who got that service.
I doubt US police asking a North Korean or Iranian service would get much cooperation.
Even for friendlier jurisdictions, Privacy is important to customers and organisations often have a legal obligation to protect the privacy of their customers, especially their identifying personal information. Police might have to invoke international treaties and get court orders which means providing evidence that a crime has occurred. That sort of evidence may be harder to assemble than many people might think. One person says they didn't receive the service paid for, the other says the money was a gift or that the service was in fact delivered, etc. This might be regarded as a civil dispute and not a criminal case.