Something unknown is all that we know. Except for those round shaped space crafts or the triangular ones, we don't seem to acknowledge anything else as being alien.
We haven't been able to progress much because we haven't been able to move any further than rockets or satellites. In a gist, communication is the barrier. Most developed societies have better communication systems. In the context of our planet also, we have seen that the countries with the most advanced communication systems have moved ahead of others. There is nothing wrong in it!
We are not sure of aliens because we cannot probe further; further than our solar system, to be precise. So, how are "we" going to know of something that only thrives in speculation, for the intellectuals among us!
Science is supposed to be impartial, un-biased, that is, based only on proof. Right it is that scientists dis-believe or disapprove of aliens but how can they disprove something for which they even cannot provide the proof of being non-existent.
How could the Vedas ( ancient Indian texts ) provide drawings of something non-existent, if at all? If you scratch the Vedas , you'll be in for a big surprise - it mentiones the different types of aircrafts, chemicals and metals used to make flying saucers. By the way, how do those drawings in the Vedas match those of the so called UFO sightings in the U.S. in the present times. There has to be a logical explanation for all these experiences.
Why is the U.S. airforce spending billions of dollars just to prove that there are no UFOs? Funny but true indeed.
Back to question- Why do aliens visit us?
It is a fact that earth has a unique type of atmosphere with a unique type of soil, absent in it's surrounding cousins and this soil contains a large amount of minerals which can be used as some type of fuel.
Might be, they have some of their generation here on earth. WHERE ??
In the oceans perhaps given the fact that only a minor fraction the oceans have been explored to the depths till now. Facinating, isn't it?
Overcrowding must be problem in other systems!
Jokes apart, The ongoing nuclear activities in our planet is a major cause for concern. It can not only blow our planet but at the same time adversely affect other adjoining systems. Remember the Hiroshima and Nagasaki ones, it was not the explosion that killed the people, but it was the after-effect that killed them and crippled the generations thereafter.
Such is the catastrophe that aliens do not want that to happen and that is the reason they visit us so often as if they are keeping a vigil. Government classified records show that nuclear war-heads have been disabled quite a number of times and word "ALIEN" is distinctly mentioned. It is just that they want us to remain in dark.
So, we are still the best at speculating!