If there have been indeed millions of intelligent civilizations capable of technology, then we would expect the galaxy to be filled with radio signals. Just look at us. We have been broadcasting radio signals for the last 50+ years.
Before long, we shall be doing most of our communication with cable, fiber, or low powered radio links, none of which would be observable by civilization on other inhabited planets. That leaves earth as potentially visible for only 100 years out of 500M years. That seems like a low visibility ratio.
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