[quote=@BingTheWing]
[s]How long has it been exactly since the Atomic Awakening? The OP doesn't really specify.[/s]

sike im a dumbass
[/quote]

I will also point out that Humanity left the surface not very long after the monsters arrived. At most they stayed on the surface for a month, as you can imagine that with the kinds of things that were popping up, this wasn't some year long struggle for survival. If they stayed very long at all they'd be wiped out. 

And within that month, that includes the time for them to get as many cities floating as they could, attempt to nuke the crap out of the monsters, thus awakening much worse things, and get the flying cloud machines going that made the permanent cloud oceans.