I think the issue is response time.

Once a drone has been spotted notifying the correct person and getting the drone started would take a few minutes I assume.

Furthermore you can't really deploy a drone at an airport while it is still active, which is the reason they are banned from operating there in the first place.

I was about to counter your first part with automatic detection and drone launching, but your second point makes sense.

From the other comments I believe that tracking to identify the operator and launch site together with automated targetted jamming are the most effective solution. I don't know if such a system exists already and if it is quick enough to react without interferring with airport operations.

if the downward facing radar sees them then it's already too late -- that drone gonna hit something or it's dropped its payload.

a couple of RPG sized rounds dropped from 10000 ft up could easily close runways and shutdown an airport for hours. hell just being there and being UFO could lead to all sorts of flight re-routing and chaos

Payload? You don’t need actual explosives to cause disruption, and for a state-sponsored actor to use them would be an act of war. The perfect being the enemy of the good here, provisioning for less hostile cases seems eminently sensible.