From the buyer's perspective you now have to go into business with some often unsavory people just to get high. From the seller's perspective you have to make sure that the buyer isn't law enforcement and also not someone who is going to rob you because you have zero legal recourse. It's really a shitty experience for everyone involved. Considering how shitty that experience is it says a lot about people and their desire to take drugs that the market continues to exist and basically never stops serving the customer.
It really seems like when confronted with something inevitable, like drug use and sales, the effort to prevent it may be more costly and damaging than just allowing it to happen in a more controlled environment.