simpler answer, they've tried and it hasn't workedthis is a serious question and deserves a serious answer
liability
first of all you cant just "shoot down" sht over inhabited areas
even a smallish drone would cause damage to your person or property and one of these super drones could wreck some stuff if it came crashing down
the gov/ authorities would be on the hook for any damage
im sure if they could identify the drone operator they would send the police out to have a chat and give them a warning about flying over restricted areas
/cant be too heavy handed
but i dont know if they can trace the signal back to the sender
i shot an arrow in the air...