while yes, I highly doubt that Blizz will be able to create a logical explenation. Not because of Blizzard but because i dont see a believable solution.
the Overwatch Universe is based on ours, just 50 years in the Future. The Omnic Crisis would bring all Nations together to prevent a Second Omnic Crisis ever happening again. The Earth today is already surrounded by hundreds if not thousand satelites. With increasingly better tech i highly doubt that even one Nullsector Dropship could be even completed without even one nation noticing it. The only possible solution would be a Wakanda move. But where does Null Sector even get the ressources from? How are they even build? I mean building such a ship need masses of material, equipment, workforce and engineering knowlege.
The old Omnic factories are either destroyed or under supervision.
And whats about the military? The Nations especially USA, China, Russia, India and NATO would make sure to be prepared for something like that. They would also have Ships like Nullsector, as i see them like the aircraft carriers of the Overwatch Future.
And it goes on and on and on. Trying to create a story of this global size was kind of a fatal error for the belivabilty/immerson for the lore.