Why don't we see robotic civilizations rapidly expanding across the universe?

In 1950, while sitting down to lunch with colleagues at the Los Alamos Laboratory, famed physicist and nuclear scientist Enrico Fermi asked his famous question: "Where is everybody?" In short, Fermi was addressing the all-important question that has plagued human minds since they first realized planet Earth was merely a speck in an infinite universe. Given the size and age of the universe and the way the ingredients for life are seemingly everywhere in abundance, why haven't we found any evidence of intelligent life beyond Earth?