[John H K]

One satellite can do the job of 1000 airships.

Nope. People have the mistaken idea that every inch of the globe is covered by recon satellites 24/7. Nothing could be further from the truth, largely because of a lot of bad movies and TV shows with real-time close up "satellite recon" movies all the time.
Actually a given location on earth is lucky if it's in sight of a recon sat for a few minutes twice a day.

[RightWhale]

And 36 satellites gives global coverage, while 36,000 airships would give no coverage because their cost would ground them permanently.