Alternatively, visitors might not be travelling forever across the void of space, they might simply be popping 'next door' as visitors from another dimension.
There was aven a B movie out some years ago which suggested that the aliens were, in fact, time travelling humans. The wizzened body we now associate with a typical alien is actually what happens to the human race after all the pollution, radiation etc.
It does seem that there has been a bit of an upsurge in UFO sightings in recent years, many captured on video.
Hardly surprising none of them land, get out & have a wander round though. A simple scan of the planet will tell them that the air is almost toxic, as is the water, not to mention the background radiation going off the scale.
It'd be like us taking the family out on a day trip to the public tip :yuck: