The "Fermi Paradox attempts to offer a few reasons why we're not seeing evidence of extraterrestrial intelligence.
Wikipedia- THE FERMI PARADOX
Kind of amazing when you think about it. There are billions upon billions of suns. Many which may possibly have habitable planets. I'm not going to discuss in detail each of the twenty one possible reasons. You can check them out for yourself at the above link. I tend to lean towards explanations 5-8, 5-19 or 5-20. If you are still interested check out this video. See what you think.