I'm surprised no one has thought of this theory before, but here's my proof...
No military on Earth is ever involved in Jasper's invasion or the literal end of the world. There's probably no nukes, since Steven Universe takes place in an alternate timeline where world wars never happened, but certainly there's super huge bombs that can be used against the hand ship. Okay... forget the military, for that might not be kid-friendly enough. What about trained engineers that might be able to build a super-powerful drill instead of a tiny one that the gems made? They're never mentioned.
Screwed up geography on maps (especially Russia).
There was, indeed, a great war that took place on Earth. What if humanity is just now starting to repopulate?
Civilization seems very bare and limited, except for one city: Empire City, which is another name for New York City. Maybe the USA was the only surviving country after some catastrophe? Perhaps because they have the strongest military? There are no other big cities shown or even mentioned, only other states and countries.
Greg says that only 15 people live in Beach City, a city that is often associated with tourism in the show. It could just be a simple plot hole, but I like to imagine it's not. Beach City is a lot like my home city, Myrtle Beach, a city that's not very populated, but gets tourists from all over the country. However, Beach City takes that to the extreme.
Bonus: There's a place in Delmarva called Dewey Beach. There's also Ocean City, which sounds very similar to Beach City.