Everyone knows which wallets the bitcoins travelled through, but you don’t necessarily know who own what wallets.
Trouble is, people aren’t very good at keeping their wallets secret.
You can’t really hack the blockchain (not easily anyway) but you can steal someone’s wallet by hacking their computer.
Of course, if you hacked their computer then you might as well just steal their bitcoins and then download a bunch of kiddie porn and send a tip to the cops.
