[robdar]

One reason is:

https://en.wikipedia.org/wiki/Treaty_on_Open_Skies

Military intelligence gathering is so common that countries basically agree to not start a shooting war for overflying each others country gathering military intelligence.

If every military intelligence balloon, satellite, aircraft, drone, etc, was considered an act of war and destroyed, the world as we know it would have ended a long, long, long time ago, and none of us would be here having this conversation.