From EndsOfDays.com:

Indeed, perhaps the biggest obstacle for otherworldly life to overcome in finding us is the survival through this critical band. The absence of visitation is thus discouraging. Let us hope that our future is not written in the stars.

This quote makes a great point. There is no “concrete” evidence of the fact that we’ve been visted on earth by an alien species. Well, there is evidence out there, but there is not enough evidence to convince the masses of earth that we are not alone in the universe.

Could this be because all life that is intelligent enough to find us has destroyed itself before it was able to do so?

An interesting thought. The human race is much more likely to destroy ourselves or at least destroy our ability to get out into the universe to find other life before we actually do so. Just look at the way things are going now. We are much more interested in blowing each other to pieces than venturing out off of the earth, at least in general as far as the population of Earth is concerned.