Why haven't we found aliens yet?

But, if extra-terrestrial life forms are abundant in the Universe - as some people believe - why have they not been in contact?

From Doctor Who to Superman, ET to Marvin the Martian, fiction has regularly brought aliens to Earth as friends or enemies but, as yet, no-one has proved they have ever seen an alien apart from on film or TV. In 1960, a radio telescope was pointed out into space to listen for signs of extra-terrestrial intelligence, trying to add scientific fact to the question "is anybody out there?"