Cost and complexity. If you have to send so many to overwhelm and bypass all the countermeasures, the cost and complexity of make it a much less practical and appealing solution than just doing it the old fashioned way. I can't see how brass, lead, and gunpowder will ever be more expensive than light weight plastics/composites, electronics, sensors, motors, batteries, plus the actual lethal component. Add to that the required time/complexity required to configure and deploy, situational considerations such as weather, sensor viability, terrain/environment factors, etc... and we're back to going back to guys with guns. Could there conceivably be a scenario where this might be the best option? I suppose, but in my estimation it would likely be the option of last resort.

If an enemy force has already made up its mind to kill, I don't see this making it any easier/more reliable/more effective than well-established alternatives.