Author and professor Timothy Ferris explained this well in the PBS documentary Life Beyond Earth with a Christmas tree light analogy. Ultimately it comes down to how long other civilizations last for and are willing to communicate. If a civilization either kills itself off or turns inward (cyberspace life, no radio outward communication) then the chances of concurrence decrease significantly.

https://youtu.be/Y_4roppaq7s?t=1h37m45s