They already do - the UK has seen drones used to deliver phones and drugs into prisons for about a year I heard. - better service than Deliveroo and cuts out having to bribe prison officers
It's not unexpected - criminals read the papers like everyone else and the drugs guys have lots of cash and can adapt whatever hi-tech they want. Remember that criminals historically bought fast cars, fast boats then aeroplanes and then fast aeroplanes. Now it's subs and shortly it'll be drones.
AFAIK no-one has gone to rockets yet - which tells you a lot about cost effectiveness and reliability I guess.......