Not all know to use bitcoin properly. There are people who regularly screw it up with their privacy, security, who they don't care, who they use bad software, who they haven't studied it a lot etc. There are a lot of examples of thieves who got caught because of these. From thieves who stole coins and deposited to CEX later on, to two folks who stored the private keys of billions worth of bitcoin in a cloud service.

Also, that's still possible: