Maybe some alien civilization does know about us, but it slightly afraid since we spec’ed in the war tree instead of space travel.
They kept letting us advance our war machines until one day they needed us. To help stop a greater evil; since they went space exploration instead of guns
Footfall nails this. Aliens show up and drop rocks on us from orbit. The reason we prevail is the only plot I’ve read that makes sense.
How To Beat Aliens
They didn’t invent the ships or tech they’re riding in and aren’t much, if at all, more advanced than us. Plus, we figure out their rigid social hierarchy and turn it on them. Also, we nuked Kansas.