Personally I think the answer is anti-natalism. Once life gets more intelligent it takes a relatively short time (compared to the age of the universe) to get to the point where they realise it's unethical to create new life and choose to go extinct.
As a result, intelligent life around the universe only exists as short 'blips', being around for too short a time to ever meet other species.
As a result, intelligent life around the universe only exists as short 'blips', being around for too short a time to ever meet other species.