Because we are in an infant phase of science and technology, it seems unlikely that civilizations would want to bother with us at this point in time. The times we live in now,(the make-or-break times if you will) determine if we can live as a race that learns to get along, or destroy ourselves.
Would you want to get involved if you saw two raccoons fighting over some food? Of course not, nothing they can do will affect your life on any noticable level, so it's not worth the time. However if those raccoons learned how to verbally communicate,and handle weapons such as a rifle, you may want to try to approach him, to establish some type of friendship, because they posess similiar technology which you may or may not find threatening. That raccoon could change your life dramatically all of a sudden.
There is no need to interfere with other civilizations unless you percieve them as some type of threat. As far as we know, we can't detect any other life in the universe. So what good would we be to a civilization that is lets say, 600,000 years old? The only reason I could think of might be natural resources. Yet, by that point in their civilization they should have any problems related to natural resources sorted out by then, there are plenty of baron worlds to mine for materials.
Another guess is that they checked us out already, maybe 5 to 10 thousand years ago. They saw we were just in our infant stages of civilization. They could have said to themselves, "we'll come back in say, 20,000 years, and if they haven't destroyed themselves by then we will integrate and enlighten them.
Or, a more far out theory could be that once a civilization understands multidimensional sciences, they upgrade and slide up to the next dimension, or a better alternate universe.
