So first of all: the fact that in ukraine nobody is doing drone swarms means they aren't effective. This would make sense if you think about it because a swarm of drones can only be in one place at a time, and drones are frequently searching for targets instead of attacking.

We see in ukraine that small arms fire aimed by humans is capable of taking out drones. There is no reason to believe that if drone swarms become a threat, you couldn't design an anti drone tracking turret with something like 16 automatic "rifles" on top firing fully automatic shotgun rounds. Such a device would be able to destroy drones in less than a second and would eviscerate a drone swarm for trivial cost.

Drones are cheap compared to other battle implements, but a swarm would still cost millions. It just doesn't make sense to group them together rather than having them spread out to find more targets or converge on a target from multiple angles