Curiosity is a key indicator of intelligence...so why have we not been visited...

. '...UFO‐spotters, Raelian cultists, and self‐certified alien abductees notwithstanding, humans have, to date, seen no sign of any extraterrestrial intelligent civilization. We have not received any visitors from space, nor have our radio telescopes detected any signals transmitted by any extraterrestrial civilization. The Search for Extra‐Terrestrial Intelligent Life (SETI) has been going for nearly fifty years, employing increasingly powerful telescopes and data mining techniques, and has so far consistently corroborated the null hypothesis.

As best we have been able to determine, the night sky is empty and silent. even with many billions rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods.
Carl Sagan once wrote that "the origin of life must be a highly probable circumstance; as soon as conditions permit, up it pops!"

There is a period lasting several hundreds of millions of years between the formation of Earth and the first known life. The evidence is thus consistent with the hypothesis that the emergence of life required an extremely improbable set of coincidences, and that it took hundreds of millions of years of trial‐and‐error, of molecules and surface structures randomly interacting, before something capable of self‐replication happened to appear by a stroke of astronomical luck.

For example, it took some 1.8 billion years for prokaryotes (the most basic type of single‐cell organism) to evolve into eukaryotes (a more complex kind of cell with a membrane‐enclosed nucleus). 1.8 billion years is a long time,
—the question then is  "Where are they?" But this view might well be completely mistaken. There is at any rate hardly any evidence to support it.
Yet  '...there are on the order of 100 billion stars in our galaxy alone, and the observable universe contains on the order of 100 billion galaxies.'  Yet   

the rise of intelligent life on any one planet is sufficiently improbable, then it follows that we are most likely the only such civilization in our galaxy or even in the entire observable universe.

(The observable universe contains approximately xxx stars. The universe might well extend infinitely far beyond part that is observable by us, and may contain infinitely many stars. If so, then it is virtually certain that there exists an infinite number of intelligent extraterrestrial species, no matter how improbable their evolution on any given planet. However, cosmological theory implies that, due to the expansion of the universe, any life outside the observable universe is and will forever remain causally disconnected from us: it can never visit us, communicate with us, or be seen by us or our descendants.
 
What are our chances of making contact with 'them' out there?

 

Considering that space travel was science fiction a mere half century ago, we should be extremely reluctant to proclaim something forever technologically infeasible unless it conflicts with some hard physical constraint. Our early space probes are already out there: Voyager 1, for example, is now beyond our solar system.


Now ths is pessimistic: '...If it is true that almost all intelligent species go extinct before they master the technology for space colonization, then we must expect that our own species, too, will go extinct before reaching technological maturity, since we have no reason to think that we will be any luckier than most other species at our stage of development.'


Source Nick Bostrom Where Are They? http://www.nickbostrom.com/extraterrestrial.pdf
Nick Bostrom is the Director of the Future of Humanity Institute at the University of Oxford. His homepage, with many of his papers, is at http://www.nickbostrom.com.