With all the Steven Universe theories out there, I noticed there’s not much discussion on the state of humanity in that world. It’s large enough to warrant the internet and there must be a large government body in place, but I don’t think human civilization is as large as in our world.
The Crystal Gems go to a lot of places with a wide variety of environments, but not one of them seems to have been touched by human hands. I don’t think you can argue “well, they’re too remote, humans can’t reach them.” We’ve been to the Moon, it’s not beyond humans to reach these places.
The only thing I can think of is that it’s Gem territory, people just don’t go there. That begs the question, though, of how long the Gems have been gone. It also raises the question of how much of the Earth belongs to humans?