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 maximum speed at which we can move has gone from approx. 60 Km/h (horseback), to several thousand Km/h.
• the maximum distance at which we can instantly communicate with someone has gome from some hundred meters (waving flags, flashing lights), to hundreds of thousands of Km. (I can now talk to anyone on Earth using my portable phone).
• we have set foot on the Moon, and have filled near-Earth orbits with hundreds of man-made satellites, some of which can look down and check what I'm having for lunch
• I can type this message and have it instantly visible to anyone on Earth who bothers visiting this site
• we have started to look at modifying our own genes, to eradicate diseases, and make ourselves more intelligent and long-lived
• On the other hand, we have also perfected an astonishing number of mechanisms to kill or harm people, ranging from the bacteriological through the radiological through the ballistic to laser light
• the maximum distance at wich we can kill someone else has gone from a few hundred meter (Napoleon-time artillery), to the entire planet (ICBM, UCAV)
• the maximum number of people we can kill at a stroke, has gone from a few dozen (think powderkeg), to millions (ICBM)
• and we have started to find ways to do all these things above while becoming close to invisible when doing them (optical and radar stealth technology)

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."