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?
Meh, Bob was having a bad day when he was assigned to grabbing an asteroid out of the Kuiper Belt and sending it to destroy life on a planet.
He grabbed the most convenient one that seemed big enough, so he could check the box, and get back to playing Missile Command.
Besides, it’ll take millions of years for any life to recover on that planet, wtf does he care, he’ll be long dead.