Memes

I personally like the ant analogy here. Imagine an anthill next to a highway. Ants will never comprehend what is happening right next to their hill. They literally don't have the capacity to understand.

Now imagine aliens are already around us but in ways too complex for us to understand. And in the same way we don't bother trying to teach every ant hill we come across our ways, aliens might not be bothered to teach us stuff as we are seen as too primitive.