At Centauri Dreams, Paul Glister has posted a new model for the expansion of alien civilizations. He makes the point that if expansion into the galaxy is remotely possible, then it is overwhelmingly likely to have occurred multiple times, even plugging in what we think are conservative numbers to the model. Bypassing arguments about great filters and assuming they are out there, this makes Fermi's question more pressing.
Glister discusses one answer, the zoo hypothesis, which is exactly what it sounds like - a "prime directive" situation where Earth is quarantined. He notes correctly that it would only take one non-cooperator to spoil the surprise for us, but then reasons that if the first aliens ever were zoo-builders, maybe they would establish a precedent. He doesn't address why the first aliens might be likely to do this, which in my view leaves this wanting as a defense of the zoo hypothesis.
A far more parsimonious explanation for why we haven't noticed aliens if they are indeed out there is our own ignorance. It requires no (perhaps provincial) assumptions about the nature of the aliens and their intentions, or even that "intentions" means anything outside of humans. But we do know that we don't know everything. It may be that we've been staring them in the face the whole time, and even if they're trying to get our attention we can't possibly understand.
How so? Aliens who have expanded off their own planet are more likely to be millions of years more advanced than us than mere thousands. This means they will not seem advanced. They will seem incomprehensible, if we even recognize them. Ever try to call your cat's name to get its attention? That's what I mean. For example: we find through an upcoming experiment that the whole universe is a kind of simulation or local physics that they've created for some ineffable purpose; and what's more, every time there's a gamma ray burst, that's their signal for us to recognize them. That kind of frightening, abjectly humbling realization is in fact the best case scenario I expect, because it means they recognize us as alive, even if some kind of interesting virus, and they care.