1. Separated by vast distances
2. Incompatible communications technologies
3. They are already here but we do not have advanced enough technology to detect them consistently.
4. Look over your shoulder...: )
Maybe we aren’t intelligent enough for them to want to communicate with us yet. It amazes me how scientists have developed all these amazing terms, measurements, hypotheses, and theorums, which are for the most part, ego stroking, and still can’t produce matter from nothing or answer many simple questions.
5) they have been here and gone many time over the past millions of years, and civilizations grow and die off faster then anyone expects (every 10,000 years or so?)