E.T. Will Probably Not Find Us in Our Lifetimes

According to this Guardian article, other advanced life forms just have not found us yet because they have not had enough time to look. According to the calculations, it may be a while before we get to say hello to E.T.

An alternative theory that I have is that once a galactic civilization advances past a certain level, it learns about an unwritten rule of the universe. That is growing civilizations are to be left alone untouched?