Why do you think we have not detected extraterrestrial radio signals?

The simple truth is that the probability of picking up electromagnetic radiation containing sophisticated information from another civilization on another planet is extremely small. While there are estimates of the number of such civilizations (Drake equation), the fact is that so far we have no way of determining the probability of such a finding when we have no physical evidence whatsoever of life on any other planet. That is, with a sample of only one planet (earth), there is no way to estimate the probability of finding another planet with intelligent species that use radio waves for communication.