That's a historical accident of arrangements on earth (so less useful in the Fermi-paradox / filter debate), and could easily have gone differently. Ie air space could also have been seen as ex-territorial.
My recollection of how it evolved on Earth was the soviet's more asked forgiveness than permission and eisenhower basically shrugged and said "whatever, at least our spy satellites can go over you too"
See also how for us earthlings the moon is considered the common property of all mankind or something like that. And that's all well and good as long as no one can actually reach it well enough to make use of it; but I predict as soon as we have useful moonbases, we will move to a conception that's closer to 'possession is nine tenth of the law', if not outright 'might makes right'.
If not de-jure, then definitely at least de-facto.