Astronomy

I mean, even if there were many civilizations, I'm betting that any that are smart enough to explore the cosmos have realized one or multiple of the following:

1. It's possible to 'explore' a massive amount of detail without leaving your home system.
2. Any civilization that can communicate over vast distances also has vast capability to deliver power to an acute area.
3. Any civilization on the cusp of 2 is capable of ridiculous levels of distruction, and it'd be wise to make sure they're totally peaceful before making them aware of you.

... Also, I'm a firm believer that humans and other megafauna are, well, mega. Humans are insanely massive on the scale of life as we know it, and life may not need to become so massive to never the less spread out. For all we know, Earth is a battlefield of life that is successful in the grand scheme of things, and humans et. al. are like the Death Stars of the ancient civilizations that are very much still alive.

(ok ok that last bit is very hyperbolic but it should still convey the idea!)