1. It's cheap.
2. It's anonymous.
The fact that we haven't had more drone terror attacks says more about the technological slowness of terrorists than its infeasibility.And eventually terrorists catch up. (Probably after the Russia-Ukraine war ends and some skilled people from both sides are unemployed)
It's infeasible to blanket every inch of civilian space with kinetic or EW anti-drone systems.
They may become commonplace at mass events (concerts, parades, gatherings, etc.) but will never cover all soft targets.
And unlike the nearest analog in chemical weapons, drones are dual-use, stable, and easily assembled.
The only reason the 1995 Tokyo Subway attacks [0] weren't worse was because of ineptitude.
Someone could be a quarter as intelligent and successfully fly an FPV drone into a target.
[0] https://en.m.wikipedia.org/wiki/Tokyo_subway_sarin_attack#Ch...