If you consider the "infinite" of space and, especially, time, I believe that the probability of the existence of (at least) one "inteligent" species capable of reaching any given place of the universe is 100%.

The question of why they don't contact us is simple: Prime Directive (http://en.wikipedia.org/wiki/Prime_Directive)

Disclaimer: I'm not a particular fan of Star Trek and bumped into this interesting principle, which I believe makes perfect sense.