> Either our mysterious alien friends really love USA

The USA is the best, who'd blame them! Perhaps aliens love American cheeseburgers and milkshakes from their time in Roswell. ;P

> you guys have some condition that the entire Europe does not, e.g., permissions to test military equipment without having to announce it to the public, let alone the freedom to move around in a large area without it becoming a political drama.

That's an interesting take. Though surely Germany or others have some more remote areas. Scotland has lots of empty land and the UK airforce right?

Then again, the amount of empty space in the wester US is not to be underestimated. I live near Mountain Home Idaho. If you drive over the desert to Nevada you cross air force land with big signs warning about it being an active test grounds. Lots of people have had fighter jets fly up and do simulated bombing runs on them as they drive through.