Just because there are no detectable radio signals doesn't mean there is no life, it may just be pre-industrial. WWII pushed us into the modern era. Without that we may still be using megabytes or even kilobytes and leaded gas. Maybe there is a planet with a single race and less aggressive tendencies, they may develop in a different fashion, while being advanced they may not be pumping massive RF for long-enough for us to detect. Not every species is going to be or was as addicted to TV and Radio as we are.