How can you take down a drone with a single bullet? It’s not trivially easy to shoot them down. Also, a “defensive” drone swarm would need more advanced tracking and targeting capabilities, so it would be more expensive. Look at Israel’s Iron Dome. A single interception costs $100,000-$150,000. The rockets being intercepted cost less than $1,000. If Israel was more evenly matched, it would be cost prohibitive to intercept all of the rockets.