Life is durable and Earth-like planets aren’t rare. So where are all the aliens?

Potentially habitable planets are more plentiful then we once thought: there are trillions in the Universe, and an estimated million in the Milky Way alone. Looking inward at our own small life-sustaining planet, we’ve also discovered that life isn’t nearly as delicate as we had imaged: lifeforms exist buried in the Earth’s crust and deep underwater, in extreme heat and extreme cold. So why, in every search for intelligent life elsewhere in the Universe, have we come up so utterly empty?