They are hard to detect. We flew 30,000 hours over the population in Afghanistan at about 2,000 to 3,000 feet and no one ever looked up. We were photographing polar bears and nothing moved. You don't normally notice.
The other thing I remember from my days in the armoured corps was that we used to fly these little RC planes and we would practise our air defence...a squadron of Leopard tanks, 19 guys on machine guns, we never hit them because it's really hard. They are small. They are only carrying a camera and it is a hard problem.
It's not that it isn't soluble, but it's not trivial.