But they’re so cheap that you can overrun your opponents defense. Who cares if they’re slow? You can shoot down any individual drone, but you can’t shoot them all down.
And if the attacker's drone swarm is that implausibly cheap, then the defender just puts up their own implausibly cheap defensive drone swarm to counter them. Checkmate.
[1] https://www.australiandefence.com.au/defence/land/more-on-eo...
[2] https://taskandpurpose.com/tech-tactics/army-xm1211-high-exp...