Kaliyuga pretty much hit the nail on the head.
As we all now that armies usually like to dress in camoflague, which makes it especially difficult to spot troops from above.
Besides, planes can't always spot infantry going at 400-700mph or whatever it is.
Besides, planes and helis nowadays usually provide anti-armor support so they aren't to concerned with foot soldiers, from my knowledge, anyway.
Anyway, this is just a third opinion.
Beware the GrimMonkey