Space, let me repeat, is enormous. The average distance between stars out there is over 30 million million kilometres. Even at speeds approaching those of light (300,000 kilometres per second), these are fantastically challenging distances for any travelling individual. Of course, it is possible that alien beings travel billions of miles to amuse themselves by planting crop circles in Wiltshire or frightening the daylights out of some poor guy in a pickup truck on a lonely road in Arizona (they must have teenagers, after all), but it does seem unlikely.
Bill Bryson: A Short History of Nearly Everything