A little while back CNN had a story about a strange creature in Panama that crawled out of some rocks towards a group of teenagers, who upon seeing it, got scared and stoned the slow moving hairless beast to death. Speculation is that it was a type of young sloth, though one with a genetic abnormality.
I think that possibly space faring aliens would theorize something like that could happen, and would prefer to not visit a panicky bunch of apes, or at least put it off until those apes became a bit more sophisticated. So they observe instead - not bothering to contact us. (Maybe UFO sightings are their way of safely testing our aggressiveness to strangers from time to time - trying to gauge by our reactions when it is safe to take the next step.)
I, Avatar, Come in Peace
Or, if they did want to visit, another reason to send avatars - robot/android explorers that the aliens could control from the safety of their orbiting mothership. No worries about germs, poisonous gasses (to the aliens) in the air, and panicky locals.
Dangerously Lost in Translation: The Difficulties Inherent in Alien Communication
Contact is fraught with enough difficulties from method of communication, to language differences, to customs (including what is considered rude, blasphemous, or criminal), that meeting actually in person would probably be not advised. Send in the avatars (or better yet, observe the sentient beings, determine how the communicate, learn it, and then, when you think they are ready, send them a message from a safe distance).