Bank robberies are detected 100% of the time, potential victims take all manner of preventive measures beforehand, and actual victims cooperate in investigations afterward. The vast majority of illegal drug transactions go undetected, and those involved actively seek out those transactions and strive to avoid investigation. Laws against consensual acts are, by the nature of the act, doomed to futilty.
What happens when drug addicts become so addled that they can no longer hold down a job? Some of them turn to various forms of crime to feed their addiction; others become homeless bums and beg to feed their addictions. If these people had families that loved them, they cause untold pain to those families who must watch them descend into living hell. Some drug addicts turn violent and physically harm or even kill others. Drug abuse is hardly a victimless crime. It may not be as overt as a bank robbery, but the damage it causes is far more pervasive and causes more long-term collateral damage.
I have an uncle who has been in prison since the 1970s for a horrific murder he committed while under the influence of illicit drugs. You cannot convince me that drug abuse is a victimless crime, or that it should be legalized.