Milky Way is estimated to be what... 13.5 billion years old. Humanity is 200,000 years old. The time span in which we are actually able to detect anything outside our own planet is what.. 100 years old? I suspect it's a lot more than a dozen times. If life is more likely to develop, then humans have only been able to look out into our own galaxy for an entire 0.00000074% existence of our galaxy. Now, let's say a civilization does not last more than 1 million years for whatever reason, that means we exist for a whopping 0.00074% age of the universe to date. I don't know if I did my math all wrong, but if correct there is an enormous chance that 2 civilzation's paths may never cross. We could be living right next to the remains of a civilization before us and never know it.