>>64498589
It was the 70s.
imagine how primitive were the chips and electronics of that era, added with costs to mass producing, while rolling the equivalent of 5 to 10 Vietnam Wars of supplies and sheit.
and yes, Shirogane Frodo-san, even stuff like artillery rounds got intercepted mid-air. A relatively bigger and slower drone would still be a gamble, even if programed to fly slow, and between moving obstacles.

For today's scenario? sure, clog the skies, but you need to erase them fast, or who knows what new strain would appear on surface...