And they did this by not not getting anything except a on off WOW signal that may be a anomaly.
This is the argument I have heard and trying to boil it down:
1 - SETI has not found intersteller communications looking at the EM spectrum.
2- The earth/solar system/sun is not unique, but common and made of common material. Since life happened here, we have to take that life happened elsewhere as well.
2a - If we invoke God in this, there is no reason for a god to create life on one planet as god is usually taken as a being of life, so then the universe is already life abundant and it would be not logical for god not to create life everywhere, but back to the basics.
3 - Evolution will eventually tend towards intelligent beings due to the advantage of using resources as tools.
4 - Using 2+3 we get that there are many intelligent extraterrestrial beings.
5 Our Solar System is a newcomer to the galactic party, so plenty of time for more advanced civilizations to have evolved.
6 4+5 is there are many advanced beings in all states of their ‘technological evolution’
7 Electromagnetic communications is sufficient for planetary communications but not practical for interstellar communications.
8 Since we established that advanced extraterrestrial beings exist and are prevalent in the galaxy we must assume they are not using EM methods of communication, and have found something better, but the only reason for doing so is the need (technological evolutionary pressure) for interstellar communications, therefor interstellar civilizations are a reality and common.
Yes I skipped a few steps, particularly between 7-8, but that’s the basic gist of it.