in reply to Extraterrestrials haven't visited the Earth yet because:

Because if they had, we would probably not be here to hold this poll anymore.

Over the last 200-odd years, we have made some significant advances in some areas :

The point of the above is not to be exhaustive, but just to list some of the things which 200 years ago might have sounded like science-fiction, if the term would have existed then (Jules Verne came later).

Now imagine a race of Aliens having a couple of thousand years advance over us (they would have to), evolved, intelligent, long-lived, cultured, benevolent (to eachother), artistic, admirers of nature's beauty, and looking like giant green spiders. And imagine that they come by around this planetary system to have a look (which given 2000 years advantage, would have to be much faster and stealthier than what we can imagine nowadays).

They might like the views and the landscape, and think this might be the ideal spot for some week-end homes and hotels.

As to ourselves however, would they look at us as possible future intergalactic chess partners (given a few additional thousand years) and leave us alone ? or would they look at us as food ? or if they find us unedible, as a dangerous pest best eradicated right away ?

I believe the first of the above to be rather unlikely. So, given that any kind of Alien capable of visiting us would have to be far more advanced than we are, the balance of probabilities must be that they haven't. Or maybe they have, but then it must have been a very long time ago.

"Visited a planetary system today, in the vicinity of X7fth56-beta. The sixth planet has beautiful rings. The third planet had liquid water and an atmosphere with oxygen, so we beamed ourselves down to have a look. The mussels were not bad, but the whole place is swarming with pesky flying things, it's too hot and it reeks like sulfur."