Individually doing atmospheric analysis for every planet in the galaxy is probably an impossible task for a civilisation confined to a single solar system. Listening for signals is something our civilisation already does. If we discover radio signals from a primitive civilisation in the next star system over there’s a non-zero chance we’d panic and try to wipe them out.
That’s the risk that dark forest theory is talking about. Maybe the threat comes from a civilisation dedicated to wiping out intelligent life that just hasn’t found you yet, maybe it just comes from your nearest neighbor. Maybe there’s no threat at all. The risk of interplanetary war is still too great to turn on a light in the forest and risk a bullet from the dark.
And while knowing this, why do we still not choose to just observe and be as quiet/ non existant as possible?
We can’t really pretend to understand the thoughts or motivations of aliens, after all, they are completely alien to us.
We can’t understand their culture, but we can understand their reasoning. Assuming their technology doesn’t break physics and they evolved via natural selection then they will have had the same universal pressures shaping them that we did. Namely that one individual can’t know and do everything so at least some cooperation is necessary to reach space and they will use math and logic to solve technical problems.
Starting with the assumptions that they use math and aren’t suicidal you can eliminate a lot of the proposed fermi paradox solutions because they propose in some form or another that an advanced race that’s also stupid just appeared fully formed. Or the whole race suddenly gets a case of the stupids.
I mean, we’re talking from the position of a race where a whole lot of us have evolved to have a serious case of the stupids and in a frightening number of cases we’ve put some of the biggest stupids in charge of running the show.
Looking at ourselves, it’s not too hard to imagine a race of religious zealots who view other intelligent life an an abomination to be destroyed but are fine leaving lesser life forms alone.