Spacecraft capable of interstellar travel are not going to be disabled by conventional means.
But even if some beings did travel across space to get here, they would probably use some atmosphere-friendly vehicles to explore the planet instead of the mothership that they used to traverse the vast expanses of space.
Flying around the world in your mothership would be like using your SUV to caddy you around 18 holes at your local golf club.;-) Illogical.
Perhaps these aliens are cheapskates and use flimsy VW-size balloon-like craft to drift around the planet. Until this week, it was a good idea...inexpensive and unnoticed by the military.