Thanks to shitty writing and worldbuilding, the Earth in SU doesnt even seem to have a competent or unified government. If Homeworld invaded in the 21st century then not much would have changed really. There's tons of alien attacks on the regular and yet we don't see a single government agency or army or whatever reacting to it, so no one in the 21st century would have stopped them except for Steven and company.
Now imagine how kino would that have been, if the US government had heard about the Homeworld gems and sent their own men in black or a black ops team to capture the crystal gems and reverse engineer their technology or whatever, and Steven had to fight with someone that couldn't be talked down. Too bad the writers are hacks and made tons of filler townie episodes instead of focusing on an actually interesting plot. Goddammit, this series truly is the poster boy for wasted potential, isnt it?