They are aircraft that is cheaper than anti-aircraft rockets normally used. One shot from anti-aircraft system cost 50-200k depending. This is great when the target costs 50 million, but drones costs start from 1k.
The Israeli Iron Dome is intercepting unguided ("dumb") rockets and shells that cost $500-$1500 each using missiles that cost ~$100k or more each. Had Hamas decided to launch 100 per day for a month it would exhaust the capacity fairly quickly. Ofc this kind of an action would reignite the war, but points at the problem with air defense systems.
In fact I don't think there's a practical solution against an enemy that has cheap, non jammable drones with a very high density, like 1 per soldier or more. There's just no way to counter that, and cheap military drones start at $5k-$10k, so it is feasible to achieve this level of density.