I have heard about two other theories that explains the Fermi Paradox. The first one is called the Great Filter (I don't know where it originated). The theory basically says that each civilisation go through different stages, each stage separated by a "filter" or an obstacle, something that is especially hard to overcome. For example, the next great filter for the human race could be interstellar colonisation. If we cannot achieve that, our specie would die as we use up all the resources of this planet. Because of these great filters, the probability of a specie reaching the stage of interstellar travel and communication is low enough that we just haven't encountered one.

The other theory is from the science fiction "The Three Body Problem" and is called the dark forest theory. It says that because interstellar communication is slow and resources is limited, when a civilisation know about another, they cannot easily identify whether they are stronger, or whether they are friendly. In that case, it is far more efficient for an advanced civilisation who is capable to destroy another civilisation very efficiently, to just destroy this civilisation upon first contact. Because of this understanding, no one who is capable of interstellar communication would actually try to reach out to another civilisation. Each civilisation is like a predator in a dark forest, hiding both for their own safety and at the same time eliminating anything that dare to make a sound.