What do you mean haven't already? They most certainly have been working on it (this video is over 6 years old for example):
I suspect these turrets are fine with a couple of drones, but what if it's 5000? What if they're autonomous so that they can fly at 1/2 a metre?
With autonomy and something to allow long-distance flight it actually challenges the whole capable-airplane paradigm. People aren't thinking like this at all.
They aren't thinking real saturation, i.e. let's say, an attack with 10000 modernized V-1s, either. I believe that 10,000 modernized V-1s could cost as little as 50 million USD. So less than a single F-35, which has no chance of even reaching more than a couple during such a saturation attack.