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?
A fair point, but even though we assume time and space are massive, there is an ordering to things.
There is a day before a civilization decides to kill it’s neighbors, and a day after.
We can assume the state of things (big old space should have had that plot arc already) but we can’t know if we are still in the opening episode, or before it.
Regarding general silence, agree that is not answered by my discussion. I personally lean more towards x factors disturbing our assumptions. (I.e. long running biospheres with zero advancement to radio age) but I increasingly wonder if we are just early to the party, as egotistical as that sounds. Imagine that the civilization that will one day rule the galaxy / universe is just now figuring out how to make a basic tool?