Because of physics: “large scale” explosives for example weigh quite a lot and require something airplane-, helicopter-, or [truck-sized](https://www.fbi.gov/history/famous-cases/oklahoma-city-bombing) to have enough lift to carry the weight of the bomb.
Airplane-sized drones do exist, but they’re not as inexpensive and easily acquirable as small consumer drones, and more importantly they’re as visible on radar as regular airplanes. Meaning that a country’s military and defense systems would be able to shoot down the “large scale killer drone” if it enters the airspace.
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