Forget visual - DIY (semi-)autonomous drones are doable today and could be pretty scary too. 4G may not be good enough for a human pilot flying FPV but it’s likely more than enough for a drone with GPS running Ardupilot to hone in on a target tagged with a planted transmitter or bugged phone.

Hobbyists have been documenting step by steps for this since 2008, nothing you’re suggesting is remotely new.

Why hasn’t it happened? What is the part of the movie plot imagination that doesn’t translate quite as well to real life?

I'm not sure, tbh, though I suspect it's due somewhat to the general technical incompetence of (terrorist & gang) organizations. Bright people aren't exactly attracted to the idea of joining an organization which doesn't value life highly, and the bright ones that do enlist may be likely to be conscripted to work on other projects or killed by enemies/rivals. It's doable by hobbyists, but doing it with the scale/accuracy needed to be dangerous is a complicated endeavour that doesn't lend itself well to this kind of organization.

Fly a drone carrying a 500gram camera behind a tall building. You’ll learn the 6 things that stops this whole idea working. It’s consigned to the movie plots for the foreseeable future.