One hypothesis states that ETCs aren’t advanced enough to contact us, nor for us to contact them. Earth might contain the most advanced civilizations in the universe, making us the only ones capable of interstellar exploration to any degree. Other ETCs could have pre-internet, pre-industrial or even pre-agricultural levels of technological advancement, leaving them unable to even receive radio signals from Earth, much less respond.
This could be used to make stories in a similar vein to the episodes of Star Trek where the main characters disguise themselves and go down to a primitive alien planet to investigate strange happenings. Maybe a war on a primitive planet is threatening a research team, and so the main characters have to go down to the planet and quietly retrieve them.
Citations:
Loeb, Abraham. “Are Alien Civilizations Technologically Advanced?” Scientific American Blog Network, https://blogs.scientificamerican.com/observations/are-alien-civilizations-technologically-advanced/. Accessed 11 Apr. 2019.
Fermi's Paradoxical Prompts 