Why haven't we discovered alien life? There's a missing element from the Fermi Paradox: life is rare because if a planet has only one metabolic mechanism, it will consume its source material until it is gone, resulting in a remaining atmosphere that is the by-product of that metabolism. I hypothesize this is why Mars is red, Venus is CO2, Titan is methane, and Europa is oxygen and water.
Earth evolved oxygen respiration and photosynthesis close enough in time for them to achieve a self-regulating balance. Self-regulating balances are rare in chemistry. Most of the time it burns out.